F-5 was designed as a low-cost, lightweight fighter. For most air-defense roles, the defending nation can afford to launch more than 1 or 2 at a time.
It was never envisioned that the plane would be in a dogfight long enough to need 4 missiles, and the low-cost radar (which didn't exist on the F-5A or F-5B) wasn't capable of significant BVR engagements. Remember, it was designed long before the AIM-7 Sparrow became reliable.