At least in theory, a 4 ohm load can source twice as much current as a 8 ohm load given the same amp. Twice the amount of current mean into a 4 ohm load means the speakers can handle twice as much power in wattage.
Nowaday, a lot of speakers manufacturers want their speakers to have impressive power handling specs to impress their potential customers therefore you see a lot of 4ohm speakers. Someone told me back in the old day, most speakers were in the range of 8 - 16 ohm. And at least in theory, a 4 ohm speakers can potentially play louder.