What I want to see as the big 4:
AJ Styles vs. Undertaker (The Phenomenal One vs. The Phenom)
AJ beats Cena at Rumble and cements himself as a/the top star in WWE, absolutely giving him enough cred for a Wrestlemania main event with Undertaker. Meanwhile, you have Undertaker win the Rumble and choose to take on Styles. Styles can carry Undertaker to a great match.
Seth Rollins vs. HHH
HHH costs Seth the Royal Rumble either by becoming a surprise participant (again), or by just doing a run-in to eliminate him. Story has been building up for quite some time now, and it's time to start solidifying this angle so we can get the pay-off.
Brock Lesnar vs. Goldberg
Seems to be a popular choice, and I don't mind it, assuming Goldberg has been putting time in at the gym, and especially with his cardio. We all want something much more than the two minute match last time. They probably each cost each other the Rumble with some kind of double elimination thing.
Kevin Owens vs. Finn Balor
WWE Universal Championship
The least likely of the 4, mainly because I doubt the WWE took the US title off Reigns only to have him not capture the Universal title at the Rumble, and also because I don't know if Balor can make it back in time. But I would love to see these 2 go at it on the Wrestlemania stage.
Bonus 5th high profile match:
Roman Reigns vs. John Cena
Inter-Promotional Matchup, Raw vs. Smackdown Live
Both upset by the fact they lost their title matches at the Rumble, they keep appearing on each other's show, disrupting their promos and matches, with the storyline being that Cena is on the way out/Reigns is on the way up. Reigns can talk up Cena being the golden boy, chosen one, etc. for the last 15 years, and it's his time to step aside, while Cena can say he'll never be replaced, you might think you're the chosen one, but you've got nothing on me, blah blah blah
What will probably happen instead...
Cena/Taker for WWE Title (Cena beats AJ at Rumble, Taker wins Rumble).
If Undertaker wins the Rumble and chooses Cena, and if Reigns beats Owens at Rumble, then I guess anyone on Raw could challenge Reigns. Not sure what storyline they'd want to follow. Odds are they'd keep the rocket strapped on Strowman and give him a shot. With the three matches they will probably have, you can afford to give Strowman a shot to see what he can do on the big stage. I wouldn't blame them...it just wouldn't be as exciting as my alternatives.