Why is the djm 900 roll effect only till 4/1??

@michel > The maximum sample time for the roll is 4000ms.  If your audio is less than 500ms between beats, then you can get 8/1, which is still 4000ms.  So what is the threshold?  500ms = 120BPM.  Likewise, if you were listening to audio at 240BPM, you'd have access to 16/1, which is 250ms.

But the djm 800 was longer right?

well I mean the roll, not the mixer....  :-)

Nope.