The Hi-Capa 4.3 has a non-adjustable rear sight. The Hi-Capa 5.1 has a windage/height adjustable bomar (?) sight. That being said Laylax (or PDI can't remember who) makes a 4.3 style fixed rear sight so it may be possible to use the 4.3 sight on the 5.1.
I've also heard of people using real steel sights but I've never seen anyone actually do it.