Instead of making the bonus only appears when all 4 pieces are worn, maybe we can have something like below:
2 pieces bonus and 4 pieces bonus
2 pieces: half effect of the full bonus (e.g. 40 force, 22 ac), 4 pieces: full set bonus (e.g. 80 force, 44 ac)
OR
2 pieces: first bonus activate (e.g. 80 force), 4 pieces: full set bonus (e.g. 80 force, 44 ac)
at least can give reward to other classes than archer and thief to get some bonus from using legendary items.