#4 AJ Styles

AJ Styles is not confirmed for the Royal Rumble, but I strongly believe he will make it to the match.
There is absolutely no way WWE will take the WWE Championship off of Daniel Bryan at this moment and if AJ loses his match, he should make his way to the Rumble match.
How Brock Lesnar ruined another wrestler's career - Watch Now!
Also, due to Brock Lesnar vs Finn Balor being on the card, Daniel Bryan vs AJ Styles will probably open the show or happen in the first half of the event. This will give AJ enough time to recharge and prepare for the Rumble match.
AJ is made for a performance like this. His amazing in-ring work, his durability and ability to get a decent match out of almost anyone makes him perfect for the iron man role.
Moreover, given the Vince McMahon-AJ Styles storyline, WWE has started, an iron man performance can help take it forward.
Styles can enter early and go all the way, falling just short. This causes him to completely snap and bring out the vicious side of him. Maybe even bring out a heel turn sometime down the line.
An AJ Styles iron man performance is something every single fan will enjoy in the Rumble and it might also start a new feud for The Phenomenal One heading into Wrestlemania.