If I were Valve, what I'd do for April Fools is to "confirm" a 4th sequel to one of their games (not HL of course). Like a Left 4 Dead 4, or Team Fortress 4. Maybe even a Dota 4. Clearly, they'd play into the rumors that they can't count to 3, so now they'll skip the number entirely.

@ivesen
They were testing out porting some of L4D2 assets into Source 2, and some people thought it was a L4D3, but it never went anywhere.