I’m inclined to suggest the OT. I’ve never used the A4 or AR, so I can’t speak on them much. The OT is a powerhouse and could easily replace most of your DAW workflow, and it will transform any gear you currently own into something entirely new. It put a big stop to my GAS because I began to view everything I already owned in a new light. If I had to sell all of my gear, I would keep the OT and my iPad and never run out of musical options.

That said, learning a single Elektron at a time is preferable, especially if you aren’t already aquatinted with their workflow and idiosyncrasies. This goes double for the OT; I’ve seen too many people on this forum and Elektron-Users throw in the towel after trying to learn the OT alongside another Elektron device. The A4 is probably much more straightforward to learn alongside the RYTM.

They are two very different devices. Do you want a poly analog synth, or a sampler/midi sequencer/sound mangler? Whatever the answer, you’ll probably find yourself buying whatever one you put off sooner than later ;).