I forgot to release the hand brake while I'm driving at around 4 to 5km. What will happen to my rear brakes?
If this was a one time occurance the damage should be minimal. If the brake shoes were worn or damaged the lining material may have cracked from overheating. This is rare if they were in good shape.
The damage done will depend on the type of parking brake brake along with the extent of wear before the incident. The older design of parking brake used the service brake shoes (the one that normally stop the car) as the parking brake, they are activated by a cable system instead of the normal brake hydraulics system.
Typically if the brake shoes were in good shape a manual readjustment of the shoes will return everything to normal. The manual adjustment consists of turning an adjusting screw inside the brake drum. Most vehicles have an access hole so this can be done without removing the drum. If your car has rear disc brakes you may have a small set of brake shoes behind the rotor that act as the parking brake. If these shoes are worn they will have no effect on normal braking.
Similar to drum style brakes these are adjustable, however the adjustment involves removing the caliper and rotor to get at the adjuster. On some Subarus and Saabs the parking brake is part of the front brake caliper, it uses the normal brake pad as a parking brake. The pad is moved mechanically via a cable. In this type no readjustment is needed, as the pads self adjust with every use.
Depending on to what degree of "on" you had it, anywhere from nearly none to possibly some. Leaving it on the last click (or two, or three, depending on how much slack there is) usually doesn't engage the brakes at all. Especially on older cars.
However, if you left it on fully engaged, then it could wear the brake shoes or pads prematurely, and cause damage to them. It would also probably start smelling as well.