Hi, if you press the left hand side button of the beat loop buttons, this will give you a perfect 4 beat loop. If you wish to shorten it down you press the button again this divides the loop from 4-2-1-0.5 etc… This should remain on beat, as it is quantized. You must ensure that your records are done with Rekordbox as I believe that it doesn't work as well if they aren't.
You can also loop in the old fashioned way by pressing the in and out buttons to the let of the beat loop buttons. These are not quantised so you will need to ensure you hit them in time. I find practising that will help a lot.
Thanks!