If a door has a smoke seal is the gap measured between the brush and the door edge or between the frame and the door edge. If there's a 4mm gape between the door edge and frame and a smoke seal is fitted won't this hinder the door in closing?

Smoke seals are designed to contact the closed door edge, that what they will stop the passage of smoke.  They should not overly hinder the door from closing, however if the seals are too tight then they can cause an over-pressure delaying or even stopping the door from closing