Having fitted uprated ARBs in the past, it is <insert major caveat> usually a straightforward job. Do it with the car standing on its wheels- if you can get it up on a ramp it's easier, but it is possible to do it just lying on your back underneath. Unscrew from drop links, remove subframe mounts, remove ARB. Insert new bar, apply subframe mounts, tighten back into droplinks. Bob is your mother's Aunt as they say. Not sure what the spanner/ bolt sizes are for the A4, anyone know?
Does the ARB kit from VAGParts come with new bolts and subframe bushes by the way? Always good practice to replace those at the same time.