Please answer. Could you use a 4ohm mixer with two 8ohm speakers at 150w each?

Answer 1

At that kind of audio power level, it's probably not a good idea to work your amp into
a 2 to 1 mismatch. The possible consequences are: > heating the speaker cables, >
lousy fidelity out of the speakers, and distortion on peaks, > getting a lot less power
out of the speakers than you expect, > damaging the power output stage of the amp
if it's solid-state (transistor) and doesn't have protection against a mismatched load.

Technically, you could connect two identical 8-ohm speakers in parallel to the same
4-ohm output connection on the amp, and all the hardware would be happy with that.
But that leaves nothing on the output to the other channel.