First things first, never listen to some terd at Numark / Akai.
They have no idea about the 4000 features vs the 2500. I had one guy on the phone from Numark when i was buying my 2500 tell me that the 4000 is less featured and that the 2500 blows it away...lol. Little did he know that i had a 4000 sitting right next to me.
They even had a nerve to change the recent mpc comparison pdf. They have now (since the lack of new 4k's) put 96 ppqn instead of 960 ppqn for the 4000's sequencer spec (Why???). This is obviously a cheap /misleading attempt to push the sale of 2500's and fooling people into the idea that both machines are just as powerful - So wrong.
In short:
4K has 24 bit 96 khz ad-da's vs 2500's 16 bit 44.1 khz
4k has a full Z-series sampler with key grouping
4k has 64-voice polyphony vs 32 of the 2500
4k has a more powerful Intel CPU
4k has 4 FX units vs 2 of the 2500
4k has many more filters for sound mangling
4K has 960 ppqn resolution vs 2500's 96 ppqn
4k holds 'note repeat' down for you - handy
4k has a bigger screen - nice and clear in black & white
4K has Aksys software for sample drop and drag to the pads in real time
4K has USB flashpen slot in front for loading and saving on the fly
4k has upto 512 ram for samples / 6 banks of 16 for sounds
4k has a 24bit preamp and grounding for turntables.
4k has ADAT capabilities
Plus others.
...Phew.
