I think thats 450 lbs per inch of compression. That would be one mighty stiff coil-over. If you're talking 4 inches of travel, that would be 1,800 lbs to fully compress it.
The coils (not coil-overs) I have on the back of mine are 175 lbs per inch. I think an equivalent coil-over would be about 250 b/c you have less travel.