I have swelling across body and have gain 4kg weight in last 4 days post parathyroid surgery under general ansthesia is this common side effect ?

Check electrolytes: After a major surgery like parathyroidectomy, significant electrolyte chnages can take place presenting as generalized swelling or swellig of the dependent parts of the body. Thyroid function should also be checked. Better see your surgeon for follow up.

See below: After anesthesia patients often see a temporary weight gain from the fluids that they received during the operation. This is compounded by the fact that most patient's need time to recover from the surgery before they can remove their normal activities. This again can cause an increase in weight, especially if the same caloric intake is maintained. This should resolve with full activity.

Uncommon post op: What you describe is uncommon for post parathyroid surgery. Thoughts that come to mind are excessive fluids given during surgery (was it long?) or an allergic relation. Getting up and moving around should help you recover.