"Cheap" and "top 4" are mutually exclusive. That said, j-bow and gunner of stl would be available, gudbranson (but Vancouver, so likely inflated price), Martinez, petrovic, outside chance at Faulk (Not really a rental, but def a need for edm; probably can't trade for him given car similar need for scoring forwards), nik kronwall+ nic Jensen. Off the top of my head anyways. Most of these guys are "capable" of top 4 minutes, very likely in the case of faulk, the rest of them are mid round picks and easily attainable for a bandage.