I have both a Mamiya 645 AFD and an F5 in my arsenal. I like them both, but for very different purposes. Shooting with the F4 will really not be very much different from shooting with your current camera except that you won't have the instant gratification you get from your rear LCD. Shooting the MF, even though 645 is small MF, is a whole different ballgame. That's what I get out when I want to go out and relax with a camera. I might spend four hours in a location, and only shoot 1 roll of film.
As mentioned, 120 film isn't readily available any more except in "real" camera stores and on-line. Processing is pricey and usually requires a lengthy wait (Typically I pay about $20 for development and a proof-sheet, and turn-around time is 10-12 business days). Scanning isn't cheap either; a service with a good drum scanner (high enough resolution to make the whole process worthwhile) is going to run $6-10 PER NEGATIVE! You can get digital backs, but they're pricey, and with a 645 (non-AFD model) you need a special (and VERY expensive) adapter plate. The cheapest decent backs will run $3-5000 used.
I keep both around primarily for black and white. No matter how good software gets, no one will ever convince me that digital images can produce as good a B&W as Delta 100 or FP4!