As a point of information, the MC402’s mono mode doesn’t bridge the two channels, it puts them in parallel. Which is actually better, as bridged amplifiers often don’t do well sonically when driving 4 ohm speakers such as yours.
Although I don’t have the user manual to look at, in looking at the rear panel diagram in the service manual (at hifiengine.com), and assuming the 4 ohm impedance spec of your speaker is reasonably accurate (as opposed to being significantly lower than that at many frequencies), I believe you would use the amp’s 8 ohm taps, and as Mike indicated you would connect jumpers between those taps. You would also set a switch on the rear of the amp to "mono parallel" mode rather than stereo mode, and you would connect the preamp to the amp’s right channel input, balanced or unbalanced as you prefer (setting the input mode switch correspondingly).
Good luck. Regards,