That grizzled piece of concept art above is the signal that Serious Sam 4 is in development at Croteam and, probably, due sometime in 2014. The developer is making use of a mid-week Humble Bundle sale to raise additional funds for the game, and offer you cheap copies of previous Sam games for good measure.
If you beat the average paid price (about $4.25 USD at the moment,) you’ll get to own basically every Serious Sam game in existence. That’s: Serious Sam HD: The First Encounter, Serious Sam HD: The Second Encounter, Serious Sam: The Random Encounter, Serious Sam Double D, Serious Sam: Kamikaze Attack!, and Serious Sam Public Test 2. Paying above the odds will add Serious Sam 3: BFE Deluxe Edition and Serious Sam 2 to that list.
Any money you designate as going to the developer will be funneled into Serious Sam 4. Judging by the concept art, he might be visiting Paris this time.