The best way is still to drop the suspension down as far as it will go (on the frame bumpers) and make sure you have clearance at that height. If it's a four link I assume it has coil over shocks? If so, take them loose and drop the frame down as far as it will go. If you still have clearance, you good to go. If the drive shaft hits the tunnel before the suspension bottoms out, you can modify the tunnel or raise the static ride height of the vehicle. How much drive shaft clearance you end up with at ride height will vary depending on your particular vehicle. My 4 link suspension has about 5" of travel(2.5" up - 2.5" down). You may have 8" (4" up - 4" down). All depends how it was designed.