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4 valves create tumble - no swirl! The combustion is related to port velocity. You want proportionately high flow with respect to the port cross sectional area. If you close a valve in A port designed for two valves, you HALVE the flow, and consequently halve the port velocity. Not only does this kill VE, kills fuel shearing and mixing, and reduces mixture motion - all of which slows flame speed, so you have to set ignition earlier, which hurts mechanical efficiency.
You can gain back your low rpm performance (and then some) by filling in the port with epoxy or aluminum solder. The port should be 86% of your valve diameter, and taper up to your valve. I can give you a figure if you tell me your valve diameter and stem dia. You obviously need a smaller runner intake manifold - and short runners (10" from valve to plenum should work well). plenum volume should stay the same. Throttle body can be smaller for better response and transient.
Fwiw an FSAE team just used a small intake manifold and ported the heads. They improved their FE by double digits iirc. Still not ideal, and perhaps easier.
Hth
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