I found out why my mp4 (and all the other format) files weren't playing and it had nothing to do with VirtualDJ.
So nothing to see here, it was just me being 'tarded. =P
I finally feel confident enough to migrate away from the windows version of VirtualDJ Pro that I've been using for 2 1/2 years now to solely use VirtualDJ Pro Mac (now with 5.2) which I've been hoping to do for... 2 1/2 years! ;) ... Actually, realistically 1 - 1 1/2 years now. =)
This whole "problem" had to do with me saving all of my content somewhere else in the windows version and the Mac version not finding that new location. All good now though!