I thought most amps can drive a 4 ohm load really but the problem is to deliver 90W to a 4 ohm load requires basically twice as much current as to 8 ohms and the wires and power supplies can be skimpy and not meant for that kind of current. So a 4 ohm load would de-rate the power rating of the amp and the potential exists for too much current to be drawn and some poping and fizzing results.
In retrospect I'm not really answering your question at all am I. Well it was more enjoyable to type then the 'real' work I'm about to have to do.
