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I have a suggestion. Don't use the extra two cylinders as a supercharger. I think that would reduce your fuel economy, shorten the engine life and give you lots of shaking at low RPM. I've tried running my Accord on two cylinders and it shook a lot when it ran at less than 2000 RPM. My front exhaust pipe cracked and my flex pipe developed a leak. At highway speeds it ran smoothly and had enough power to maintain speed up most hills.
For smoother operation and better fuel economy you could try using the two deactivated cylinders to expand the exhaust gas more before releasing it into the exhaust pipe. At full throttle, the pressure of the burned gases should be about 30 PSI above atmospheric pressure at the end of a power stroke. Sending that gas to two cylinders for expansion to double the volume before release will recover extra energy without using more fuel. If you pump a little water in the passage to the expansion cylinders, you may get some extra pressure by making steam.
When the load on the engine is small, it might be better to release the exhaust gas from the operational cylinders directly into the exhaust pipe. It won't do any good to expand the gasses until there is a vacuum. That would waste power. One way to solve that problem is to use one way valves between the exhaust valves of the expansion cylinders and the exhaust pipe. The gas from the expansion cylinder would not flow until the pressure in it exceeded the pressure in the exhaust pipe.
Ideally, it would be best to let the two expansion cylinders switch to normal operation when extra power or low RPM was needed. That would require some switching of valve timing and gas flow into and out of the cylinders.
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