Penn? Pens? Playpen?
Reminds me of this XKCD:
http://xkcd.com/150/
Odd "grownup" references aside, could be something to do with fences or walls? Or contained areas?
Maybe even a raised container above the field, something like this but a bit sturdier?
http://www.target.com/A401010-Balloo...dp/B000Z55P1O/
4? 4 teams? Maybe back to a 2v2 game?
Doubles tennis?
Or maybe something to do with the felt texture of tennis balls?