Difference between S4 and S5 control arm?
 

I bent my control arm slightly on Saturday and I have 2 other RX-7 parts cars which are both series 4s.
When doing the switch I noticed they are physically different; the S4s look noticeably more "beefy" than the S5.
Are the two OK to use interchangeably?

they are interchangeable.

the S4 arms are preferred because the balljoint is removable/replaceable.

when the S5 balljoint wears out you need to throw the entire assembly into the garbage = waste / high cost of rerplacement.