The 4* ground-in advance is to help those guys who don't degree a cam, and just stab it in with a stock type one-key-way gear set.
Sometimes due to parts variation the ground-in advance helps when using a 3 keyway gear set by allowing you to gain a little additional advance for variation adjustment and for future chain stretch.
If you are degreeing in a cam with a proper degree wheel, etc. then where the cam is actually ground is irrelavent.
Decades of experience has taught cam grinders and users that MOST combinations will produce more power and "feel better" when the cam is installed about 4* advanced.