I suspect that the gain from going to 4 valves with the Jumo 211 would not have justified the disruption to production. There would have been a much more worthwhile gain with the faster running Jumo 213 and it would seem to be a good question as to why the Jumo 213 did not go to a 4 valve cylinder from the start. It may be that they were hoping to get the Jumo 213 into production with minimal delays and wanted to avoid fundamental changes, though, as we know, there were significant delays anyways.