Cut salmon into 4 equal pieces; place in shallow glass bowl.
Mix soy sauce, mirin, sake and sugar in small bowl; stir to dissolve sugar. Pour marinade over salmon; let stand 10 minutes. Drain salmon, set aside till required, reserving marinade.
To fry: Heat oil in 8 – 10 inch skillet over medium heat.
Add salmon; cook until light brown, 2 to 3 minutes.
Gently turn salmon over; cook just until salmon is cooked through and flakes easily with fork, about 2 minutes.
Reduce heat to low.
Add marinade; cook just until salmon pieces are well coated and sauce is hot, about 1 minute.
Serve immediately with pan juices from skillet or remaining marinade, if desired.
This is fabulous on a bed of udon noodles and a fresh green salad.