Not sure where you got the idea that you would be able to upgrade as long as you had the points. If that were the case no airline would be able to sell revenue tickets.

Airlines allocate seats for redemption/upgrades, the number depending on route. As sbs2716g said, flights to Japan are in extremely high demand now so they literally do not need to give out 'free' upgrades. With your four seats requirement, that would be even harder, no matter the airline. You can be placed on waitlist, but they don't tell you your position in queue so you can only hope they let you know weeks before the actual flight.