You should be fine for awhile with four. No heavy off roading.
If you have four locking lug nuts, you should have the four regular nuts they replaced. Even if you bought the locks with the car, the dealer should have put the take offs somewhere in the car.
Just be sure its the nut and not the stud. It's best to leave it off if either is stripped.
Don't panic, but replace it as soon as you can.