Could I use 4 ohm in place of 8ohm speaker here? - Model Railroader Magazine

Cisco Kid

Would I be able to swap in a higher quality Zimo 4 ohm speaker for the 8ohm default one?

No, not unless the SDN144 specifies it can use a 4 ohm speaker.

I wish to use just one speaker in my new VO-1000.  Would I be able to do it with additional electronics added....

Since you don't want to use a second speaker, you could get another 4 ohm inductor to put in series with it.  Something like this. .  Note that I am not recommending this unit, just using it as an example.

What I can't tell  you, without knowing a whole lot more about the speaker and doing come calculations,  is how to match the resonance frequency of the inductor with the speaker to prevent it from acting like a low-bypass filter. 

Since you say your not good with electronics this probably won't help.  All the on-line sites I can find that are related are for building crossovers.  This one has a "first order" section which is actually the most simple  Butterworth crossover.  The inductor would be L1.

As the others have said, the best thing is to find an 8 ohm speaker that meets your size needs.  There are bunches and bunches of them out there.