I explained this on another thread - I agree it's a pain!
It's either written as fifths engaged or fifths palpable. Palpable means what they can feel. So 1/5 engaged means that 1/5 of the baby's head is in your pelvis, so on up until 5/5 (fully) engaged when the midwife or doc can't feel the baby's head any more. 4/5 palpable means that they can feel 4/5 of the baby's head, meaning 1/5 is engaged. So...
4/5 engaged is further along than 1/5 engaged
Alex - your GP is using "fifths palpable" and your MW "fifths engaged".
Hope that makes sense!