the analog four and keys can sequence external gear via CV only. you can sync external gear using MIDI, but that is it as far as the MIDI control goes. for the CV control, you can send pitch information (either V/octave or Hz/V), triggers, gates, and LIN values, all depending on what you are trying to accomplish with the external gear. see the manual describing the CV outputs.
with your example, the volca keys supports two ways of external control. the first is MIDI in, which supports both pitch information and syncing. the other way is via it’s sync inputs and outputs, but these (as their name suggests) is only for syncing clocks and not for accepting or sending pitch information via CV. so with the analog four or keys, you would only be able to sync the clock on the keys to the analog four or keys and not send pitch or sequence information.