Can an exported MP4 count as a copy of my original?

Only when the pair is confirmed. Media Organizer confirms a pair itself when the export has the recording's name and starts and lasts the same; it suggests the others for you to confirm, from Exports to confirm in the library report. Once a pair is confirmed and turn on exports counting as a copy for that camera family, the export counts as a copy of the original on the drive that holds it. See Companion files and exports.