Maybe this will help...
The 4K can read the current AKP programs that have been made for the S5000-S6000 and the Z4-Z8 samplers. The 2500 might be able, yet it is not listed (via Akai comparison). Therefore the 4000 creates keygroups naturally. Yes the 5000 can do this, via OS upgrade only. If you use multisamples and want to recreate a keyboard, or what ever, it makes more sense to use a machine that supports it from the ground up. Many "MPC-purists" don't like it. Yet, it is a very good option to do this, via the "Multis" function, similar to Performances loaded with Patches on Roland equipment, as well as have your drum programs at the same time. By the way, in the MPC-Resources section of this site, there exists many newer programs that will now support many of the older MPC and S-Series samplers.
The 4000 also has 6 pad banks. That is 96 samples pads, 32 more than every other MPC, more than the 500 can see (though I like my 500, it's good to 'go' ). So, if you set up the muiltisample program per midi channel, with this machine, you have lots of drums, samples or phrases you can manipulate. Imagine 16 midi channels, times 88 keys (just specialized pad banks at this point). That would be 1408 pads at you command. That is how I viewed my Akai S2000 when I got it before I ever got an MPC!
Akai S-2000
http://www.vintagesynth.com/akai/s2000.phpOkay, so you can write quick music "masters" to the CD in the 2500 versus the 4000. Yet, the 4000 can have the 8 outs and the Adat out board installed for more options. I have the wordclockdigital out, 8 outs installed on mine. It also has the same SCSI-port installed like the MPC2000/2K-XL. RIAA phono input standard, XLR and quarter inch standard outputs. Two USB ports (type "B" to use with the AkSys software, one standard 2.0 port that you can connect card readers, zip, hardrives, etc.) I've upgraded mine with a internal IDE to internal SATA bridge that allows me to use my Seagate Barracuda 1TB internal harddrive. Yes one-Terabyte, one-thousand Gigabytes harddrive. A portable studio if you asked me...
The 4000 supports more of the Akai libraries for input. I don't think the 2500 has as many.
Yes, the 4000 has many built in options. So you'll have to download the manuals, and all of its revisions from Akai. JJ-OS might give more, but take some options away, on the 2500. At this point it is all about how dedicated you will be in learning about the machine you've invested in. As always, read the manual.
I have both machines, so I can skin an animal in many different ways with these, and the other machines I have (look at my signature). But, It all boils down to your preference, your diligence and the depth of you pocket when it come to making that hit of your dreams, or making the beats that every group will seek you out for!
Peace and good luck!!