I hope I'm not leading you up the garden path here, but my understanding of it is as follows...

The GP feels for bub's head and measures it in fifths. 4/5 above the brim means that bub's head is 4/5 above the pelvis and 1/5 below. Obviously, the more fifths below the closer you probably are to the birth. Having said that, I know that some women's babies don't properly 'engage' until they are in labour. Everyone is different.

I hope that's right! Please, anyone, feel free to correct me/elaborate on what I've said if I'm mistaken.