The VR-only Serious Sam game, The Last Hope, is still in Early Access, having recently added co-op. But though it's not officially "out," that hasn't stopped developer Croteam from hinting at their next project within the game.
According to Dark Side Of Gaming, Sam's in-game gun is engraved with the word "PATENTED" along with the following six dates, each of which correspond with the release of a Serious Sam Game:
March 21.01 (Serious Sam: The First Encounter)
February 2.02 (Serious Sam: The Second Encounter)
October 11.05 (Serious Sam 2)
November 22.11 (Serious Sam 2: BFE)
October 9.16 (Serious Sam: The Last Hope)
December 7.17
The last date, then, assumes the next game in the series will release in December of this year. Whether this corresponds with Serious Sam 4 or another game in the series remains to be seen, but chances are high we'll get something from Croteam by the end of the year.
Our Take
It's very likely this is Serious Sam 4, but I find it weird Croteam is already teasing their next game. I guess this adds the feeling that Early Access titles are considered "released" as soon as they hit the service, and not when they leave it.