I used a 3" axle because 4" would've put me below the "scrub line". The scrub line is a straight line from the bottom of the rim of any of your 4 wheels to where any of the 3 remaining wheels touch the ground. (NSRA specs). You can get more than a 3" drop from a 3" axle by either removing leafs from the spring pack or going with mono-leafs. Wheel size obviously figures into this also, I'm using 15" wheels, with 16" wheels, the 4" axle probably would've been o.k.
