Whats up Boogie, Doveski.
I've never used a Z4 or 8, read good reviews and complaints mostly about crashes and clunky effects. But if what you're looking for is a slick platform for grabbing samples, the sampling automation and program layout automation are killers. They speed up the process of putting together nice multisamples quite a bit from all I've read. Can't speak for the quality of hardware, though they do seem a bit "plasticky" next to older S series boxes.
I've owned in the past an S2800 (dug it, was confused at first by the multiple loops per sample concept) and currently own an S5000. (which of course only has one loop per sample, though multiple loops did allow for some neat tricks) The 5k is a real nice piece if you can snag one with a USB add-on. Connect to ak.Sys (which has given me more than the occasional "hardware disappeared" type bogus message but it's a nice free program for building programs and multis), more memory, great loop-creation assistance and a nice screensaver feature which turns off the screen after so many minutes.
One thing I miss from the 2800 was that it booted with 4 default samples loaded (a sine, square, triangle and saw wave if I remember correctly) which I always tweaked and played with before loading my own sounds. Also, older S boxes can transfer samples via MIDI using programs like Recycle, whereas Akai's newer beasts all require the use of ak.Sys.
Anyway, there's my rundown of some of my personal knowledge rack samplers. Also check out emulations of rackmounts in software like the 2 samplers in Reason, Giga, all that jazz. I've only played Reason's samplers but it's crazy what computers can pull off today.
Peace,
-tank.
You don't know what I know
Cuz you aint seen through my eyes
This spliff is always lit but it's a natural high
http://www.dcbeats.net