Decided to record proof for shits and giggles.
4.06 is not an optional update. The spoofer no longer works either. I was also on 3.55 for a long time before I decided to update because I actually want to play some new games. Basic internet features also worked while I was on 3.55.
Games that do not require a higher FW will work fine on old FWs, even updating those titles will be fine. But like every other system (included the prior ones from Sony, PSVita and PS3), if a game requires a higher FW, it will ask you to update; however, it will not force the update on you.