MyBB, is free, easy plugins install, super moderator category exists, in IPS 4 doesn`t exists. For me is important, a moderator moderate any section of any forum, super moderator moderate all forums, must be a separate group. In IPS 4 a moderator cand wand ans suspend the admin (hm...), in MyBB, not, a mod can`t warn and suspend a super mod or admin, I think is better so. But you must secure strong all ! So that nobody can hack the admin account (if this happens, is better that every moderator can suspend the admin). And admin must not have acces to database, for security, when the account is hacked, data base remain ok.