The four ohm tap will have twice the damping factor of the eight ohm tap in a properly designed output transformer due to it's lower output impedance. In many instances the four ohm tap will yeild tighter bass response into all speaker loads, however at reduced power transfer if the speaker is an eight ohm, or higher, load. If the impedance curve of the speaker tends to peak where the in-room frequency response of the speaker dips, then the lower damping factor of the eight ohm tap will cause a peak in the frequency response of the amp causing the overall response to sound better. So, yes, try both taps.