Why does my 4x4 not work on my Mazda b4000?

Hubs need turning on?

Or maybe a fuse?

If it is an automatic 4wd system (i.e. does not have manual hubs), you may need to replace the hubs.

To diagnose this, jack up the front of the truck and place jack stands under both front axels so that the wheels are just off the ground. Then jack up the back of the truck so that all 4 wheels are suspended off the ground. Start the truck and put it into (forward) gear. the rear tires should be spinning. engage the 4wd system. Look underneath the truck. If the front drive shaft is spinning, but the front wheels are not turning, then you need to replace the hubs. I am told this is a pretty common problem with these trucks.