How much good does the breastplate actually do?

I'm riding a horse on some very steep trails with my English saddle. At the top of a hill, I'll notice that the saddle has slid back about 4 inches or so. By I'm at the bottom of a long steep hill, it's back where it should be.
I'm considering putting a hunt-style breastplate on him.
Here's what I don't understand. When adjusted correctly, I should be able to get my fist under the breast-ring and under the wither-strap. This makes it loose enough that by the time I'm at the top of a hill, my saddle has still slid back 4 inches. At that point, the straps are now all so tight that they're probably restricting his movement a bit.
What am I missing? It doesn't keep the saddle in place, and the whole point of adjusting it with some slack is so it doesn't restrict movement. Isn't it?

A breast plate is designed to prevent minor slipping - a four inch slip tells me your saddle may not be fitting the way it is supposed to. I'd re-check your saddle fit and perphaps consider trying a saddle pad designed to reduce slippage.