S4 & s5 afm
 

is the only difference the size? does it use the same plug? or would I have to change that.

Totally different. The S4 is a flapper door style, the S5 is a hot-wire (I think that's what it's called) type. The plug and wiring is different. I remember someone once figured out how to wire an S5 one into an S4, it was overly complicated and definitely not worth it.

haha thanks for the clarification! I was curious about it.

the S5 is not hotwire..it is still technically a flapper style AFM.

the plugs are different, it can be spliced to work (only difficult if you think splicing 6 wires is difficult)

there's no real advantage in doing this.