you need a different banjo bolt -- the 2 piston calipers use a long one that goes side-to-side all the way thru the caliper body. The 4-pistons use a short one that just go front-to-back.
Having done a swap from 2 piston calipers to 4 piston calipers for a customer, what olslash said is correct in that the banjo bolts are different. However I believe that all new shimano calipers come with the correct banjo bolt for the corresponding caliper style so it should be an easy swap over