As far as I understand from the OP, he wants to be able to run 2 cabs from his amp to shift a bit more air. Unfortunately he's bought a 4Ω cab & the amp's min load is 4Ω.
The idea of an attenuator isn't ridiculous, but there's better options as other's have said. 1, get a series cable or 2, sell/swap the cab for an 8Ω jobbie.