I dont get it...? Why people when talking about the next pc worms game, call it worms 5? as i know there have been more than 5 worms games on the pc.... just curious....

The number represents the game engine: for every new game engine made, the number goes up one.
For example, Worms2 was about the 5th worms game made, Worms3D was the 8th, and Worms4 was the 10th. At least, I think so.

Except Worms 4: Mayhem is based on a heavily improved version of the Worms 3D engine...

The answer to asl's question would be because Worms 4: Mayhem was the last game in the series released for the PC.

Personally I don't think the PC needs any new Worms games, though, so I think I'll be off to the WA forums now.

w4 was a bit of a misnomer, but it's there, so anything new and different would have to be w5. officially or unofficially.