The FZS has a reserve fuel capacity of 1.5 liters and the minimum 4 liter fuel requirement is just a rumor. The fuel quantity does not alter your performance and condition of the bike as only the problem occurs when the running bike encounters an empty fuel tank. That can cause some bit of a problem as the engine won't get a proper fuel-air mixture. You can have a refill each time you get a reserve and even its safe to fill within next 30 kilometers.

Quick Facst - Yamaha FZS Version 2.0