I opened the following Stack Exchange question: https://civicrm.stackexchange.com/questions/49150/trouble-with-cv-api-version-3-and-4/49151 and it looks like this is probably an issue with cv's implementation itself.
The objective is to get (for example) the email address for every user in a give group. Using api3, an argument like options='{"limit":100}' seems to be silently ignored.
Maybe this could be fixed? The reason I'm using api3 in the first place is that (as mentioned in the question) api4 doesn't seem to know how to query groups at all.