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 frying pan 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 flavour and help to keep the salmon moist. Serve once the salmon is cooked through and is easily flaked with a fork.
If your local supermarket doesn't stock sake or mirin, you will find them in Chinese/Oriental speciality stores.