Start with your compression tester. Do a 'dry' test (all plugs out and throttle wide open) - record readings for all 4 cyclinders. Then re-test 'wet' (tea-spoon of engine oil in each pot). If all dry readings are around the same, and not significantly down on your wet readings, then things are looking pretty good in terms of rings, valve seats and head gasket. Big difference between dry and wet on one or more cylinders suggests rings. Down compression both dry & wet points to head gasket and/or valve seats. You need to know compression specs for your engine - not one I've worked on, but info easily to hand suggests something around 170-180psi (with modest standard wear - even 160psi may be be Ok - again you need to find the full specs). Your leak-down tester can help narrow down exactly where air is escaping from assuming you have a problem.