Can you explain why my drone was hovering at around 4 feet , but the altitude on the screen said 8 feet

Because the barometer that the Phantom uses to measure altitude is not 100% accurate.
You will probably find that heating of the Phantom during a flight affects it's accuracy.
You need to allow a safety margin when making altitude calculations.

It would also happen if the ground level at the hovering point was 4' higher than where you took off from.

Always take the altitude reading with a grain of salt, I have noticed times it looks like its way off on altitude by 10+ feet or so and other times it's near spot on. Most of the time I just eye ball it and adjust altitude as needed.

It went up to its normal 4 feet mark from takeoff and whilst hovering their i was checking the screen thats when i noticed the difference.
Calibration was done a few mins before takeoff and all seemed fine.

It wasnt an issue as i always air on the side of caution , just strange as its always seemed to be right

dayviduk Posted at 2017-9-24 14:39
It went up to its normal 4 feet mark from takeoff and whilst hovering their i was checking the screen thats when i noticed the difference.
Calibration was done a few mins before takeoff and all seemed fine.

A 4 ft discrepancy is not at all strange.
Check your indicated altitude at the end of a flight and you will often see bigger variances than that.
Within 10 ft is good, some users report quite a bit more than that.