Hi Gilera,
Bit difficult question to answer on the other hand... the A4 got already 4 outputs (CVs/gates), so why not taking this the easy way and start to work with your A4 (Elektron Analogue Four) as the unit that controls your modular system and save for the moment the money for a MIDI to CV module.
If you then after a while realise that perhaps those 4 outputs aren't sufficient then you still can start to use your OT (Elektron Octa Track) via MIDI and with a MIDI to CV module. At least if the A4 seems to be sufficient, you save yourself the trouble of finding a good MIDI to CV module as well as the money for it :-) And by the way... if you want to utilise your OT fully... you need at least a module that can handle simultaneous 8 MIDI tracks... most modules only can go up till 4 tracks though, so you need already to MIDI to CV modules, saving even more money if your A4 is sufficient enough. I therefore would try to start this of with your A4 first and see if that's enough.
I don't know the Hexinverter Mutant Brain module very well but from the information I have read, looks like you also can go up till 4 channels simultaneously, perhaps with a few tricks a few more channels, not sure. Another module to consider and that I am using is the Vermona qMI 2, a very nice logical module to use and up till 4 channels, each channel: gate out, 1V/oct, and 2 CVs out, beside that also clock out. I had for a short while the Doepfer A-190-5 but somehow I didn't feel that module was logically to use at all, difficulties with the menu (understanding) and I exchanged it for the Vermona, and that was so much easier and better. It's a pity because I actually do like Doepfer modules. I got quite a few Doepfer modules and I am happy with them, the A-190-5 is so far the exception.
By the way... if you got already an A4 and OT... why do you still need modular? You got already two lovely nice devices :-) !
Kind regards and good luck, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads