Personally I like to see at least 3 cylinders per turbo, so a twin turbo 4 cylinder doesn't make sense to me unless you can do something tricky to make it work well or for some reason it makes sense for financial or other odd reasons. Variable geometry turbos could maybe make up for a lot of the problems, but they are very expensive right now and if you have a properly sized variable geometry turbo then one of those would be perfect for a four cylinder and there would be almost no reason to use two of them (unless the single turbos on the market don't flow enough air for your huge power goals).