I think it's Ludwigia peruviana (Peruvian Water Primrose). I have some photo's in the database that I took at the Merritt Island National Wildlife Refuge last year:
Peruvian Water Primrose (Ludwigia peruviana) It's listed as a category I invasive here in Florida so I'm going to pull it out!
edited to say: I read that this Ludwigia can have 4-5 petals!