Whilst it is probably possible to re-save older .mx4 files from Mixcraft to version 5 .mx5 files using the standard File ► Save as.. function, the opposite downgrading of the .mx5 files back to .mx4 seems to be highly unlikely.
Same extensions are also used by MAXQDA data analysis software which is backwards compatible, hence you can easily convert between .mx4 and .mx5 files.