The Athlete feat (PHB, p. 165) has a point that says:

climbing doesn't halve your speed.

The Centaur's Equine Build trait (GGtR, p. 16) says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

This question has nothing to do with the difference between listed climb speed and walking speed.