I've considered swapping engines for a slightly smaller one cylinder. Hadn't really thought about the reasons why it would be more efficient till you asked. I'd rather go electric though
FYI, more cylinders can make an engine LESS dependable/reliable because there are more moving parts.
And there are 3 cylinders that have had the same gearing fitted as the 4 cylinder metro, and thus they have the SAME cruise rpm. And I'm guessing they cruise more efficiently because they are smaller displacement and have less cylinders.