I think all four will have situational roles but dont see any of them getting more than maybe 200-300 non-special teams snaps unless there are injuries to starters.
Long is going to have a hard time seeing the field other than special teams but I am sure there will be a plan to get him some reps. Same with Rapp due to Christian being a good 3rd safety. Maybe that changes as the season progresses like with Johnson in 2017.
Henderson will likely be the most visible of the rookies early on but it will be interesting to see how it all shakes out by January.