Melt the butter in a large heavy frying pan over medium heat. Pour in the lemon juice and swirl the frying pan to mix the melted butter and lemon.
Place the 4 fillets, flesh side down in the frying pan for 3 minutes, or until browned. Turn fillets over and cook for a further one or two minutes, depending on the thickness of the fillets. Spoon lemon butter sauce over the top of the fish as it cooks.