8ohms into 4 ohm cab?

Yeah, my guitarist friend bought a head+cabinet, old Acoustic head and a peavy 4x12. The cab said it was 4 ohms, but when i looked onto his amp, it had a switch selector for 8 ohms or 4 ohms, i quickly flicked the switch and told him that it would work better. But, could he damage his amp by running the head in 8 ohm mode eventhough its a 4 ohm cab?

 

thanks