I have the HF leakdown tester. It works fine. The only trick is establishing the baseline pressure is like steering a big ship. If you're low and you increase the flow too quickly, you overshoot your mark, and vice versa. You can also let it run "wide open", which can allow you to listen/observe the air leak if it is in the valves.
Head gasket is a likely culprit, however, I know in my case and other folks on BCF that the head gasket failure tends to be between cylinders 2 and 3, not 3 and 4 (or is it 1 and 2? I always forget which way to start counting).