Using wood blocks to increase Jack lift height?
 

I have used wood blocks 3-4 inches high in order to jack up my SUV's/ Pickups in the past. Is this bad practice? Or should I spring for the correct SUV/truck jack with 21 inches of lift?

I do of course use proper jack stands when working underneath the car. I prefer to use ramps whenever possible also.

Wood blocks are fine, in the 4" range you are proposing, as long as they are stable and not likely to topple. Position them so the grain goes across the load path, and not end on.