for sure in shallow area a 7 mm will not compress much therefore you will be more + buoyant for a wild. Mmm floaty feet ? This look more like a trimming issue. Heavier fin will not solve the problem. 7 mmm wetsuit, thick boot, thick wood, thick glove make a lot in regarding the buoyancy.6-10m. Next dive in march is 15m. 7mm wet suit. My fins are slightly negatively buoyant (I have floaty feet so instructor gave me heavier fins). Gear rented (only my own mask and hood), my last dive where these issues happened, the shop forgot to fit my BCD to me, so it was a mess probably contributing to my float up.
You should check again if you are weight properly by doing a weight check at the surface. ( bit difficult to explain ! check YT ! ).
ohh i re-read you post, are you diving in salt water ? If it s the case 4 kg is not enough while using a 7 mm.
Be safe