I know of a couple who got married in the local registrar... and then flew out to do the big "wedding" with family/friends.
It's the thought that counts, and the specific legality was really not that important to the bride compared to a wedding ceremony which will make other people turn green with envy.
Four Season Samui, btw, is a beautiful resort from my impression while having dinner/drinks on site. It's not my kind of resort, but I will say it's probably a fantastic place to stay if the FS style fits your travel profile.