A quick search for h1 bulb in exterior forum turned up this: Parts list
H1 bulbs (Hi-beam)
H7 bulbs (lo-beam)
A quick search for h4 bulb in exterior forum turned up this: h4 vs 9003
"H4 bulb is intended for motorcycle and off road vehicles. It's design has a heavy duty coil to withstand vibrations, whereas the 9003 coil is lighter and intended for automobile use. The bulbs are interchangeable."
H4's will work in the focus....H7's are for some aftermarket lighting, I have them in my Halo's...as far as the actual difference I think it is just the amount of light and the heat that it puts off....Peace