Can you post a small image of the result you want to reproduce?

Of course, how about this:

Question 1. This is the first question. Answers are laid out in {multicol}{2} format.

   A.                                  C.                       

   B.                                  D.

Question 2. This is the second question. Answers are laid out in proposed {choiceshoriz4} format.

   A.                B.                C.               D.  

My school requires a format that always places choices A,B,C,D horizontally in this pre-set position. Choice A in this format would always be flushed left, aligned with choice A when one uses {multicol}{2} or choices format. Choice C would be aligned with choice C when one uses {multicol}{2}. Choices B and D would be evenly spaced. For, example, choice B would always be exactly halfway between A and C. The idea is that everything looks nicely aligned when exam is printed.

I imagine this setting would depend on the page size and wouldn't be very flexible, i.e. one would always have to write four answer options. However, this lack of flexibility is not a problem in my specific case.

Perhaps you can use the multicol LaTeX package, and enclose the choices environments inside multicols environments:

\begin{questionmult}{pref}    
  Among the following cities, which ones are french prefectures?
  \begin{multicols}{2}
    \begin{choices}
      \correctchoice{Poitiers}
      \wrongchoice{Sainte-Menehould}
      \correctchoice{Avignon}
    \end{choices}
  \end{multicols}
\end{questionmult}

I've tried this, it works perfectly and looks very good when printed!

So, the trick is to change parameter {2} to {4}, i.e. {multicols}{4}.

Thanks a lot for the solution. :)