KIN TUT wrote:and that enables a loop to play at any tempo ?
Generally you wouldn`t ever want to speed up more than 25 bpm and slow down less than 10 bpm witout getting artifacts. That is the nature of timestretching or change on hardware. To do any big changes on audio you would have to slice finely down to at least sixteen notes which on any mpc would come out choppy sounding.
That is any mpc.
Now if you use Ableton Live 6 or 7 software you can easily set warp markings down this small and smaller and export the tempo changed loop to your mpc.
If you read the faq and the manual I post you can see what the 4k can do. You sound like a beginner ok. I recommend you pick up MPC tutors MPC2500 bible or a great book The Remixers Bible at Amazon.com. Research as this is basic knowledge also. From your questions I gather that the 2500 would be the better choice for you.