Following Mozart or Beethoven, you don't need a coda, you just go straight to the next variation in Major. If you really want Variation IV to return to F major before Variation V, why not just use a Piccardy 3rd on the last chord?
You're also asking how to get to F major from C-sharp, but you're not actually ending in C-sharp. Your D-flat chord at the end of your score is just a VI chord in your key, but you haven't actually modulated because you haven't defined the D-flat as a new I chord (with, say, an Ab7.)
However, if you did modulate to D-flat -the key- instead of D-flat -the chord-, I would probably go
Db Gb64 (aka Gb/Db) C7 F