There is NO reason to have "friendly" or "easy" robber in arena play. Those are settings intended for games with your friends in which you want to institute "house rules" or in games with beginner players, which is the
opposite of arena play.
Further, as many have stated, 3-player Catan is also pointless for arena play. There are enough "quality" spaces to permit people playing BINGO rather than Catan. The number of trades are drastically reduced because all players can grab 6/8/9 (or something similar). The point of arena is to be competitive, not to play a fancy version of "Can't Stop" or "Yahtzee".
There are plenty of arena games that require 4 players to start, and they do. Catan, which likely has one of the largest groups of players, would have no problem.