An AlNiCo 15", Altec 421 or a JBL D/K 130 or 140, would seem to be in order, but I don't think I've ever seen a 4 ohm one. Maybe a pair of 8 ohm K110's in parallel in a smallish vented cabinet? I'm not very familiar with Fender amps, but there almost has to be an 8 ohm tap on the output transformer that you could use for a single 15" cab.