Serious Sam developer Croteam has not said one single thing about the ongoing development of the fourth Serious Sam-game for quite some time now, but in an interview with Reboot Magazine (interview in Croatian, but texted), they have reassured fans the game is being worked on.
Apparently Serious Sam 4 makes use of some cutting edge tech. Things like motion capture and photogrammetry are being incorporated into the engine and will be used in the game.
Thanks, DSO Gaming.