Mach is the microkernel, the base level of the operating system and the one which interacts directly with the hardware. Mach was originally developed by the computer science program at Carnigie-Melon University and was used by NeXT Computer, the company Steve Jobs founded after the 1985 spat with John Sculley, Apple's then-CEO.
BSD stands for Berkely Standard Distribution, that is, of UNIX; Darwin (and, hence, Mac OS X) use FreeBSD, a free variant of BSD (duh) as an intermediate layer between Mach and the rest of OS X.
The rest of OS X consists of the Aqua interface, AppleScript, QuickTime, Quartz (the 2D graphics engine of OS X), OpenGL (the 3D graphics engine of OS X), etc...
SO...
BSD 4.4 is more than welcome, as long as it's completely stable, which I happen to know that it is. Apple will integrate and improve everything and wrap it into 10.2. Go Apple!