Maybe there were two different types of heater assembly - one with four screws securing the intake housing and one with three.
Here's a pic of an assembly out of the car - it obviously had three screws.
The only screw I can get to is no1.
The other problem is the front flange of the housing sits in a deep recess in the soundproofing which means it has to be lifted away from the bulkhead first (by about an inch) before it can be slid away leftwards from the heater assy.
