Regardless of the model of 747, 5 tons is very little fuel
But it is more than Reserve Fuel which, by definition is; "Fuel to hold for 30 minutes at 1500ft above aerodrome elevation in ISA calculated with the estimated landing mass on arrival at the alternate or the destination, when no alternate is required". As 4468 points out, this is more than might be on board after a diversion.
Airclues