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Without combustion happening to aid ring seal, you may get a lot of oil in that cylinder. Once it starts back up you might have trouble with emissions from oil burning in that cylinder. Revisions to ring design and PCV design might be necessary.
Without combustion but with the air still coming in, you are not only asking the other cylinders to run the engine, but there is still a compression stroke on the uncharged cylinder. That is the main reason why if you pull a plug wire on a 4-cylinder engine (or even an 8) it runs so pathetically or barely runs at all. Unless you disable the volume of that cylinder by deactivating the valves, you'll counteract any fuel savings by having to supply the other cylinders with more air and fuel to compensate for the work its not doing.
Basically, the engine takes lets say 16 hp to maintain idle. If you drop two of the four cylinders injector pulse, you've reduced its power capacity to 8 hp, but the fact that there are still four compression strokes means it still requires 16 and it would stall. What you would have to do is have the computer open up the throttle a bit and supply the two running cylinders with more fuel. Since you're now still making 16 hp to support idle, you've not saved any fuel, and in fact may have lost some. If you disable the valves, too, you're reducing the hp requirement to keep the engine running. Then the other two cylinders are only supporting their own compression, plus the minimal friction of the other two.
Caddy's 8-6-4 was a pretty good idea, but in the end it didn't save much fuel at all. The cylinders are dropped during periods of lowest load. Since fuel consumption is at its lowest during those times anyway, the amount of fuel that is saved then made little impact on the overall MPGs.
I think you would get a better increase if you recurved your ignition and fuel. Most factory ECMs keep the car a tad rich to prevent detonation and to keep NOx emissions in check. Dropping a little fuel and making sure the ignition curve is as advanced as it can be without detonation will pick up a couple MPGs on most cars. In the process, you'll also pick up some power.
Or convert to a diesel
