Ive used the MPC4k last week after having owned an MPC3k for >10 years.
3 Things i noticed:
1 The timing / groove of the sequencer is indeed different. But the MPC4k has got 960ppqn, which is great for flams, and musical sequences. I prefer the MPC4k on this part. You can also move midi notes on a very small scale, emulating more human syncopation, or even go wild with it, offbeat like Flying Lotus, Madlib, Prefuse74 (which is much harder to do in the MPC3k).
2 The sound is very different. The mpc4k sounds more thick and robust in the high frequencies. Especially hihats, shakers sound just great, very hard to get the same level in a daw. It might even be worth the extra work of tracking all high freq percussion in the daw.The weaker side of the mpc4k is really the low end. Kick drums don't sit too well in the mix easily. I noticed that the same sample kit i used in the mpc3000
which got me great results, didn't cut true the mix on the MPC4k. Somehow kicks got burried, like the lower mid punch is missing. Don't get me wrong, kicks still sound real, and robust, but it just doesnt have that natural punch the mpc3k gave me. Overall i still prefer the MPC4k sound because musical samples (Chords, stabs, vocals) sound great, i'ld easily use the 4k as a surrogate harddisc recorder for synths.
Buy a mpc4k and a secondhand s3000 puts you in a much powerfull position than the mpc3000
for a lower price. TIP!!!!!!
The interface is totally different, the visual note editting in the MPC4k is worth the step down in stability. THe mpc3k was totally bug free, and felt like the os of a nuclear powerplant,,,
People who hold on to the mpc3k might hype it up. But the most honoust opinion i can give is that the MPC4k is overall, the winner. (More versatile, more ready for modern production, more horse power).