Can I change Invisalign trays in 3 or 4 days, if the next tray fits me fine?

Acceledent and Propel are fairly new to all of us.  Each patient is different too.  It's really hard to say.  As a previous answer stated, the discrepancy may be very small and not noticeable but if, over time, you end up with certain teeth not tracking, things can get out of whack.  If the lagging tooth or teeth make the aligners subtly not fit as well as they should this could mean that none of the subsequent trays fit as well and the next thing you know, none of your aligners fit properly and treatment stalls.  We don't really know a whole lot yet about just how fast is OK.  Remember that just a few months ago, two week aligners were the norm and we thought going to 7 day aligners with Propel or Acceledent was amazing!

It is totally normal for the tray to start to feel loose on the 4th or 5th day. Basically this means that all of the movement that was built into that tray has been expressed. This is a great sign! If you change trays too frequently and all of this movement is not achieved, then eventually some of the teeth may stop "tracking" or fitting properly in the tray. This is something that you may not feel in the next few trays, but eventually there may be some teeth that are falling behind in the prescribed movements. 
Your doctor would know best whether you are able to move on to the next tray any sooner.
Best of luck! Keep up with the great compliance!