Yeah, it would probably always override the mirroring. There aren't any mapper controls related to it that I'm aware of. Though I do recall seeing something like a 'one-screen mirroring' feature for the MMC1 (I think it was MMC1).

As nice as a feature that it is, games that use 4-screen are very uncommon. In fact, the only north american release I know of that uses it is Gauntlet (MMC3).