Would it be possible to do a little cutting and drop a set of 5.25's in there?

If not, here are a couple sets of 4" I found

CDT Classic
http://www.woofersetc.com/index.cfm?...Product_ID=869

Diamond D3 4.5
http://www.woofersetc.com/index.cfm?...roduct_ID=2850

Imagne Dynamics
http://www.woofersetc.com/index.cfm?...Product_ID=909