Mix the soy sauce, sake, mirin, and sugar in a shallow dish; place the salmon fillets in the dish, skin side up. Cover and refrigerate 2 hours or overnight.
Heat the vegetable oil in a wok or skillet over medium-high heat. Drain the salmon and reserve the marinade. Pan fry the salmon 4 to 5 minutes per side, gradually adding the marinade to the pan, this will intensify the flavor and help to keep the salmon moist. Serve once the salmon is cooked through and is easily flaked with a fork.
