The shutters for my 90° source 4 look curved even though they are straight. Is this normal?
Yes, this is normal. It is difficult to design lenses that will produce a 90° beam - and the curved shutter edges are normal with this optical system. The only way to fix this situation would be to create an optical system with many complex lenses. This would lower the light output of the fixture and be much, much more expensive than a normal lighting fixture.