I bought a 92 f150, the previous owner replaced the 4x4 push buttons to a stick. How do I shift into 4x4?

 

It depends on what the previous owner reinstalled in the vehicle.

To activate 4 WD you need a transfer case. We don't know if he left it in there, or removed it.
If you don't know what a transfer case looks like ask any shop or auto store , they should be able to look under your vehicle and tell you.

Before going any further , let's see what you find.We'll take things one at a time.