The only way I would convert a 4x2 to a 4x4 would be if I had a 4x4 V8 Trac sitting next to my 4x2 Trac. I would do the 4x4 swap at the same time I do the V8 conversion.
If your looking to do minor off road and beach stuff , You can get Bigger tires and a higher lift on a 2wd any way... AND with the wider tires you can do trail stuff and minor mud stuff with the 2wd.....