It’s 3 per channel max. So you could wire in a fifth on one channel. However, if your amp is in stereo mode, that one channel will be playing through 3 speakers, whereas the other only two. This can be balanced out by switching to mono.
AMP doesn’t mind having more speakers on one channel as long as you don’t go beyond max for either channel.
With respect to left and right you could alternate with adjacent speakers. (L-R-L-R-L in a string) With this arrangement the stereo image will be reversed in some locations, but everyone can have a stereo image. Or, as jgatie suggests, switch AMP to mono.
Might be able to adjust the balance control enough to avoid rearranging speakers or going to mono.
Still you might want to try mono anyway as you might prefer the more even sound field it provides in an open area over stereo.