I have re-calibrated my IMU a couple of times and the controller but after flying for a few minutes my phantom 3 hovers at 4' above the ground and the remote shows 15'. Is it something that should be expected from a cheap drone or is that an anomaly?

it must be an anomaly. as mine shows the correct height.

Is it the same after you restart your aircraft and APP?

Let DJI-Tim help you by answering his question.

DJI-Tim Posted at 2015-9-17 03:00
Is it the same after you restart your aircraft and APP?

Tim, I am having a similar issue.  See the thread below. I set up a waypoint mission and it is saved.  Evertime I fly the mission and then come back around and fly it again by the time I get to the end of the 2nd pass I am very nearly at ground level.   I have all points set at 35' AGL.   2 days in a row now it has happened.

http://forum.dji.com/forum.php?mod=viewthread&tid=31825&page=1#pid224760

DJI-Tim Posted at 2015-9-17 02:00
Is it the same after you restart your aircraft and APP?

I could re-create that behavior like clock work. I got tired of it and sent the drone back. Waiting for it to be exchanged. Not a good start with DJI. I hope it gets better.

DJI-Tim Posted at 2015-9-17 02:00
Is it the same after you restart your aircraft and APP?

I just got a brand new Phantom 3 Pro and I have the latest firmware on it and on the remote and I have the latest app and I have calibrated the IMU on a kitchen table away from metal and electronics and it's still dropping altitude while hovering. We are talking 1-2 feet per minute. And the app shows the same altitude. It doesn't drift side-to-side... just the altitude.

For me, this is a serious problem.  Once the P3 is more than about a hundred yards away, it's easy to lose perspective and misjudge height.  For example, I want to fly it over water (some nearby lakes).  However, I don't want to fly it into the lake!

Similarly, I want to slowly fly around an object using the POI mode.  I don't want a corkscrew pattern.

Or filming in the rolling fields near here, with the harvester going.  I'd rather not end up making an unscheduled landing.

I can understand and live with minor fluctuations; a few feet.  But I am experiencing drops of 40-50 feet in the course of a flight.