Pix4D has provided an EXIF editor tool in their desktop software
essentially, make sure you have a confident number for the absolute take off altitude
export your exif data to file csv
open this into excel
find the correction for your first photograph, and apply the same correction to each photo altitude number
save the new column values over the top of the original column, and delete the wasted data
re-save
open EXIF data from file in Pix4D, then import the replacement data