You say you have 4 lines running to the MC, one for each wheel.
I'm assuming that means this MC has 4 independent ports.
In that case, after you bleed one (or more) of the wheels,
that last one can't be bled properly. The back pressure
of the other independent ports are keeping the brake pedal
from pumping pressure into that last line.
In a case like that I would recommend one of those
vacuum bleeding tools to remove all the air from the last line.

I have never seen a MC with 4 independent ports.

Pooh Bear