It is nice to see some of the earlier Android apps that are still good being updated.

Here is what is said on their website:

With the latest updates our apps RD4 Groovebox and SPC Music Drum Pad for Android are now Pro Audio feature ready. Improvements in Android 6.0 Marshmallow now allow an audio roundtrip latency below 20ms on Android devices which support this feature. The Pro Audio device feature gives our apps excellent response time, comparable to other well supported pro audio platforms.
At the moment devices we know of that support Android Pro Audio are the Nexus 5X, 6P, 9 and the Nvidia Shield Tablet K1, we hope more devices to come soon. For the technical details on what the Pro Audio device feature consists of, read this article on the Android Authority website.

RD4 being one of the few applications that have full midi control it is nice to see someone using the midi keyboard controlling some different parameters of RD4.

Playstore link:
Mikrosonic RD4

Playstore link:
RD4

Mikrosonic is on a roll!
They have stepped up their game and as a result they are one of the only two ones 
to have an Android device that are fully midi integrated!
The two coolest things you can now do with RD4 is that you can control RD4 with a midi keyboard which is not new but now you can control all the parameters of RD4 with the controllers you have on your midi keyboard! 
The second great thing is that this also goes for your DAW. So now you can use RD4 as an external synthesizer controlling, automating and sequencing it directly from you computer!
This makes it very attractive for people that uses a DAW and like RD4.
(There is one other application that is also recently have midi integrated in this way that I will soon write about... just want to try it out first.)
It is very nice and hopefully this will be a standard as it is a good thing to be able to compose and control your favorite Android synthesizers directly from an external sequencer / controller.
To do this you are going to need a device that can use a OTG-USB cable for your keyboard/midi controller. 
One more extra goodie is that it is even possible to use a midi to USB cable if your keyboard do not have USB connection... 
To read more about this go here:
http://www.mikrosonic.com/rd4-midi-android

Playstore link:
RD4

And here you have the Update list ripped directly from Playstore:
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- USB MIDI support, connects external MIDI keyboards and controllers
- External MIDI clock sync support
- Automatic MIDI input focus when switching instruments
- Detailed MIDI Implementation Chart here: http://www.mikrosonic.com/rd4-midi-android
- Improved low latency support for devices with a 48 kHz sample rate such as the Nexus 4, htc One, Samsung Galaxy Tab 3
- Support for x86 CPUs
- Improved Delay and Filter effect
- Analog synthesizer: Performance improvement
- 8 bar loop export fixed
-------------------------------------------------------------------------------------------------------------------------------------------------

Using four of the new synthesizers that came with the last update.

Saw this one thanks to Palmsounds.net

Playstore link:
RD 4

Well the music sucks and does not give justice to the application but if you have not seen RD4 before it is interesting  to see the possibilities and then you can read the review here at Musical Android- 
http://www.musicalandroid.com/10/post/2013/07/mikrosonics-rd4-review.html

Playstore link:
RD4 Groovebox

So not completely made with RD4 but the drums and baseline yes. Posting this because it does not get much music posted outside of Caustic / G-Stomper / SunVox and because I said yesterday that there is not much music made with vocals... and had forgot about this one.
and actually he made another song some months ago that was good but the singing was too off key for me to post it- here it is still slightly on the limit,  but better.

Playstore link-
RD4

This is the new RD3HD that used to be two 303 emulations and a drum sequencer containing ten drums sets- Would say that of the 303 bass line emulations I preferred this one as it was easy to manipulate on the fly which is as it should be, the idea of it being a groovebox application.
Now they have released a new version of RD3HD and it is  a such a big update that the name is now RD4 and deservedly so. It is a true pleasure to play with as was the RD3HD but now you can use four Sound generators which can be mixed as you please, it could be four 303's if you wanted- for example, but the exciting news is of course not this- 
It is the fresh new Synthesizer module. 
Not too complicated but having enough options and power to tweak out a big range of sounds that will keep most synthezists happy.
You can have up to three oscillators that can be up to six different waveforms and tuned separately so just with these two options you can go far. And in with the three oscillators tunable separately  you can create three note chords which can come in handy as the only critique that I have with RD4 is that the new Synthesizer is monophonic- 
maybe this will be addressed in a future update?
It also comes with the usual suspects- Envelope/Filter and for live tweaking a very handy and responsive pitch control.One last thing is a control of Mikrosonics own invention going under the name "warmth" the idea is that the more warmth you add the more analog synthesizer feeling will be added. What it really does is  fluctuating the sound like some kind of minor instability/vibrato added to the sound  the more you turn the dial- 
It is interesting and I like it but if the sound feels warmer I guess is subjective.
Then there is also a very nice and responsive pitch control that I think is perfect as it is placed just right for easy pitch control.
It comes with the possibilities to record notes live which makes it fun to try out ideas instead of being bound to putting notes into a sequencer.Think that it works better and is more fun than FL Studio Mobile which I like just because of the same reason- the ease with which you can record and try out new ideas on the fly. If the Latency is  a issue you can always go into the piano roll sequencer that comes with the new Synthesizer module and nudge things in place. 
To be honest this is secondary to the true purpose of the application as it is meant to be played in the way that a groovebox would be played- 
To sequence notes and to manipulate and play with the different parameters. 

There is an area where some people will be wary of RD4- there is no programming of the automation you have to record it by hand and so if you want to make a whole song it is necessary with some practice beforehand- I do see this a plus as it will force you to a more creative approach. 
There is also the nice effects that you can manipulate with the X-Y controller and all this is perfect in portrait mode on a tablet as almost all of the controllers are reachable and responsive from effects to parameters of the synth to fucking up note sequences in the Bassline module and changing drum patterns by clicking buttons etc.

Inevitable there will be some comparisons with Caustic having somewhat similar modules and at a first glance looking kind of similar- But it is not the same thing. Caustic being meant for composing and have more possibilities in sound design and also have deeper sequencing but would be more frustrating to use for live playing/tweaking- while as mentioned RD4 is for live playing and could easily be part of a live set up. Or even as the star of the show if you set it up and rehearse before hand. They will live in harmony side by side in your Device.
It all depends on what your inclinations are or what you want to do.

Recommend this application and think that even though by Android standards it is slightly "expensive" being around 5 $/€  it is easily worth it and it will definitively be a main application that will be reached for again and again and can not stress this enough- 
How fun this app is to play with! 
YES! 
Now there is one more Android application that is whispering for me too come and play, keeping me up at night.

Frank Malm

Playstore link:
RD4