What a rich question.
I love RY4's and they are so different.
I'll start with my favorites...
BWB and nHaler's R Y Not. They are similar... much heavier on the tobacco, light on the caramel... and the vanilla really comes through. I would not call them sweet -
I also like Janty, fresh dekang, Dulcis and Taboo. If you loved the way the pre-filled carts tasted when you first got your PV, then try these. They will delight you.
A few very unique ones...
...because of the awesome vanilla punch, BWB has a very "different" one.
FSUSA's is really unique. I have different views on it than some. I find it extremely sweet and rich and earthy. I don't get so much tobacco from it - while others do. It's a good taste... heavy in my opinion... worth trying.
In my opinion, BWB's espresso is pretty close to FSUSA's RY4... though the espresso has coffee in it while the ry4 doesn't.
Vaperite... because Todd uses that fresh leaf tobacco, VR's has a smoothness to it that I haven't had anywhere else. A lot of people seem to like this one.
Vapor Stations RY4 and VS4... sweeter and worth trying.
also, if you like FSUSA's I would give Purevape's PV4 a try. different taste, but similar complexity and similar flavors in different ratios... PV4 is golden while FSUSA's is dark.
I am sure I left some off... lots of ones worth trying.
...and while it isn't RY4 in any sense, many RY4's aren't RY4's either, so I say Try Alien Vision's Boba's Bounty...