The best player to replace Rooney in case of injury or suspension is none other than Carlton Cole.
His style of play closest resembles that of Rooney among all the options. He brings power and a relentless will. He's a good finisher, has speed, and good assist #s for a striker.
I think Carlton Cole is a fine choice.
I'd bring Rooney, Defoe, Cole, and Crouch...
If a 5th striker is brought, I'd check Agbonlahor's form at right wing. If he's dangerous there, like he was several years ago coming up with Aston Villa, I'd like the versatility he provides. But otherwise, Bent would be my 5th choice with the consistent form he's shown in the league this season.