The info I got so far was that Act 4 is "quite advanced", but requires more work. On the other hand, you stated that BP2s next release contains Act 5 as well, which was, to my knowledge supposed to have multiple endings, each path probably consisting of more than just 1 mission. Aside that you said several times that the basic mechanics in Act 4 and 5 would be similar and hence take less overall work, there wasn't a single comment yet on the status of Act 5 AFAIK (unless you were simply referring the entire time to the 4+5 combination without saying it), which could be anywhere from "roughly the same state as Act 4" to "maybe I or the rest of the team finds the time to built 10 or 15 missions from scratch".