A lot of folks like the four wheel steer models - so many so that many two wheel steer models have been converted to four wheel steer.
To a purist (not that I know any of those
) this is herasy. If this is going to be concern to you, better to know on the front end that after the purchase.
Though I have no personal exeperience, it is my understanding that parts for the four cylinder engine are much more difficult to find than those for the two cylinder.
Send a PM to Chuck W, DavidB or Maccus on this site - they all have restored VERY nice Mules and they should be able to give you first hand info.
Hope this helps,
David Doyle