Most of the 4x4 vehicles have instructions written on the back of the sunvisor of driver side. Do chk if it says anything specific on your vehicle.
In general in most part time 4x4 vehicles, if you are going in straight line upto the speed of 40 kmph, and your tyres are inflated to approximately equal pressure all round you can safely engage 4x4 on the go, even if you are going on bitumen road. However turning wheels on bitumen road or any road with traction will put lots of strain on driveline and can even damage. On surfaces where there is less traction, like mud, snow, ice, sand etc you can engage 4x4 at similar speeds even if you are turning a bit. On lesser traction surfaces like above you can drive at higher speeds with 4x4 engaged.
Obviously, all of above is assuming your front hubs are locked, either permanently or intentionally.
A tip, I would suggest that to avoid any sudden stress / jerk, just lift off the accelerator pedal briefly while engaging the 4x4 lever if you are engaging it while moving
For engaging low gear 4x4, one should stop the vehicle, put in neutral and than change from 4 high to 4 low. In fact in auto equipped rigs, its necessary to be in neutral (not drive, park or reverse) otherwise your gear wouldn't engage in 4 low. hope this helps.
