So to clarify they both would protect from an opponents mayhem and
Rain becomes dust?
4DC does not protect at all
from Mayhem, because it is not a discard ability.
Simon the Zealot protects completely from RBD (as long as they, say, didn't steal it from you with Abe's Kid, because then it is not an "opponent's card" because they don't both own and
control it). Your deck is completely protected from it, nothing can be done by RBD if it is your opponent's.
4DC partially protects against RBD. It does not protect against the "instead reveal", as only the discard is protected. So, any cards drawn by a special ability you control "are instead revealed". All Lost Souls are placed in territory. The rest are supposed to be discarded, but they are protected from that. Revealed cards that don't have another destination are returned to where they came from, in this case the top of your deck. So, RBD vs 4DC leads to the drawn cards being revealed, LS being put in play, and the rest returning to the top of deck.