Asif, to engage the 4x4. first you need to press the Hub Lock Button, it should be on the right hand side of the steering wheel. then press the H4 button next to it. Now your ride is in Hi-4WD.
To disengage 4x4, first press the H4 button then the hub lock button.
If you need more power (useful only in extreme offroading conditions) you can engage Low-4WD. you can do this by first locking hubs with the button and then use the small Gear shaft to shift to L4
Your 92 prado should also have a knob on teh left bottom side of Steering which says something like Off-RR. this is a diff lock knob. once you start doing some serious offroading you will need this. i won't confuse you with it at this point.