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.
@endlessmike Those are fake rumors though. They've been developing game all along. The games just don't live up to their standards, so things rarely get released.