Single or Double entry fee for Badaling Great Wall

We like to take the North Cable Car up to tower 8 and then (roundtrip) return by Cable Car to take the South Cable Car up to tower 4 and then walk up to tower 7 and then (roundtrip) return from tower 4 by Cable Car as well.
Do we have to pay 2 times the entry fee in that case? And how long does it take to walk from tower 4 to 7 and then back (give or take).