Can't properly answer the poll since I go much higher than CR + 4. Honestly, I haven't gone as low as CR +4 with a real BBEG since the halcyon days of party level under 8th. But then, real BBEGs in my games tend to be rare; usually what ends up happening is that the party makes a mess in one adventure, and dangling plot threads lead to further troubles they need to clean up later on. So they're more reacting to events and trying to keep a lid on genies they themselves let out of the bottles, rather than trying to foil plots laid down by some ancient dragon, nasty wizard, or insane godlike entity.
When they do face a true BBEG, that creature or character is extremely powerful compared to them at the start, and it becomes the focus of a whole series of adventures. Think Adventure Path here- or at least Adventure Arc. So where the BBEG's CR is in relation to the party depends largely on when the party gets into position to confront that entity, and is variable.