Compression numbers can get pretty low and still cause no issues. Most cheap gauges aren't even that accurate. You could see an even 110 all the way across and have no problems even though 120 is considered low. Normally you are looking for one abnormally low or an adjacent set low. It's pretty common these days for people to simply screw up the test so be sure to read up and do it right. The 4.0 is nothing special, everything you read about compression tests applies pretty well to this engine too. You don't need a 4.0 expert.