Case 4a: voltmeter not rotating
Brush connections allow voltmeter to remain stationary while the magnet and conductor rotate. EMF is measured by voltmeter (whenever the conductor rotates to the right position to connect to the brushes, of course).
Case 4b: All equipment is rotating
Since the total emf generated around the complete loop of conductor and voltmeter is zero, the voltmeter will not read any emf. This is because, even if the voltmeter is far away, it just means that the reduced influence of the magnet is balanced by the increased length of conductor.
Case 5: All equipment rotating with flux path modification
The field path is looped round the magnet in two directions using soft iron or similar.
Of note is the conductor route which exits at the axial point, at the centre of rotation.
In the region of the field detail (shown at the North end of the bar magnet), the conductor exits from the high field concentration where the field lines are not crossing the conductor. At this point, emf is not generated along the length of the conductor. Outside the main field area, there is little field to speak of, so the emf generated in the conductor is small, compared to the emf generated by the conductor at the end of the bar magnet.
The question is: would the voltmeter show an emf if it rotated with the conductor and magnet assembly in this case?