At the very least this should be done at the end of TS4 development and production as a standalone, as somebody mention here I agree with: the singleplayer needs priority of improving. IMO the multiplayer/online could in theory work, If done well. I do play and enjoy playing FreeSO, but I do find a lot of room for some improvement.
I do agree TS4 Multiplayer/ONLINE (as a standalone) would have to a M/+18 rating. For obvious reasons thirteen age players shouldn't even be playing that sort of game. I do have an open mind, but as long it's executed at the right time (end of TS4 and as a standalone game without locked/exclusive features that feel fit in singleplayer as well). Besides, dlc are still being produced and release and multiplayer would then be complete (perhaps for those who own the packs for singleplayer will be able to fully access the game or have certain advantages of it to multiplayer component).