You have a four channel amp and four 4ohm speakers, so one speaker per channel would get you 4ohms per channel @ 75w each.
Bridging is if you only want to drive 2 speakers with that amp, usually two 4ohm subs (or one sub with dual voice coils). Each of the resulting two channels would then see 300w. As it is unlikely that full-range speakers can use that kind of power, four speakers would sound better than two, and may even be louder despite only receiving half the wattage.