I'm sure it's not the 4WD; as mentioned in other threads, the 4Runner's 4WD system can be used any time, even on dry pavement. Even if it was an older style 4WD, the 4WD system has nothing to do with the ignition.
Sounds like a basic mechanical problem with the key cylinder. I don't think there are any safety gadgets that would prevent the key from going in. The safety things would prevent the car from starting.