The fourth number is the L1-L2 height difference. Again, all values are in meters. The units are degrees C, millibar and precentage of saturation respectively. This parameter effectively controls the detection of cycle slips. If a change greater than this rate is detected, this change is treated as a discontinuity, i.e. a cycle slip.
Small values are appropriate for quiescent data from sites with well determined coordinates.
Larger values useful with noisey data or poorly known sites. The difference between the polynomials from opposite sides of the cycle slip at the cycle slip effectively gives the correction needed. This number will change with the observation interval. When using a 30 second observation interval, this implies 10 points. This gap serves as a flag to subsequent programs that a new phase ambiguity must be estimated.