The disputed four southernmost Kuril islands are worthless except for national pride. Russia also has no aims against Japan — If they wanted to conquer any more land than they already have, they’d rather have a warm weather Indian ocean port through Iran, which was their strategic plan in the 18th century. Right now, though, they’d rather get stronger economically.
The Japanese consider these islands important for their fishing industry. Also, possession of all the Kuriles effectively makes the Sea of Okhotsk a Russian lake.