@Joachim, i recommend Future Controller Quad! It is the hottest fin setup for the sweet potato right now (in Hawaii). I seen a bunch of guys riding 4'8, 5'0, 5'2 etc with these controllers rippin it up. Another option would be an all flat foil quad set (e.g. Scarfini FX3 or FX4). A cheap and effective setup that I used when I didn't have money to buy fins in the beginning were the Futures T1 twin fins paired with some 50/50 foil trailers (you probably could use flat foil too). Btw, I used these fin setups on a 5'2, 5'4 and 5'6 SP.

I heard the SA4 Simon Anderson were good too but have not tried them.

The sweet potato is very floaty and loose and that is why I went with fins no smaller than L. My other recommendation would be to sell your SP and get a baked potato!! haha. Have fun!

ALOHA!!

I know little about Futures, especially their quad trailers but Controllers may feel a little big for you. I'd consider WCT fronts with some 50/50 quad trailers.

Have you looked into the Scimitars? I started using them on a thruster, and have them on my SP. They seem to 'drag' less than others I've tried and the board turns on a dime. Here's a blurb: "FUTURE FINS SCIMITAR 451 Vector Tech Foils were developed for the specific purpose of creating maximum speed and kinetic drive through the turns. Based on the physics that create lift on an airplane wing, the fins produce high power (lift) with drastically decreased drag. All the Vector series fins combine different force foils in one fin outline, one at the base of the fin and one in the tip... With the blunt instead of traditional rounded tips these fin move a lot of water providing for a powerful fin system. This set combines three separate planes creating three angles of attack, and curve inner faces for unreal water flow. Drag is reduced dramatically, and your board will feel faster and looser. The design of these fins focus lift where the demands of your water pressure are highest-from down the line drive at the base to the tip for fin releasing turns."
A lighter guy would use smaller rear fins...

I like the Controllers in small surf with a little power and the scimitar w Stretch quads when really grovelling little inside shore breakers
Scimitar plus goes rail to rail better for me .
When I've gotten some decent set waves on a more solid 3-4 ft wave the Cont def have a ton of drive but I like the more shortboard feel of the Scim pl Stretch