...items to other players in multiplayer is planned, but how will it work? Will players use a similar single player style, 4 items carried to next floor, or will players be able to carry 16 items (4 items * 4 players)? Will players share the same pool of items that they can immediately grab from during combat?

That's a good question, I assume it'll either be the pool method or single inventory but you can trade the items, end up being like the resources that way, or in the slot where people get rid of there items that could be like a team inventory and people could grab what the others don't want. I honestly don't know. But I have faith in Amplitude that they'll make a wise decision.