If you have a vacuum leak the o2 sensor would be reporting a lean mixture. The ECM would respond by adding fuel until the o2 sensor reads correctly again. This process is known as Fuel Trim.
Short Term Fuel Trim is the ECM's corrective response to a rich or lean running condition in real time.
Long Term Fuel Trim is the ECM's corrective response to Short Term Fuel Trim.
This would not explain why your engine is only running on cylinders 3 & 4. You must either have no spark on the two non-firing cylinders or a loss of compression, which you said earlier was good.
Does the inlet manifold have a divide which separates cylinders 1 & 2 from 3 & 4? If so could 1 & 2 be starved of oxygen? Does the running of the engine improve or make no difference if you spray brake cleaner or something similar in the air intake? You can rule out an ignition fault if it runs on all four when you do?
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