If you're considering an upgrade from an MD4 some minor spacer work will be needed on your motor mounts to raise the motor slightly.  With the larger transmission gears of the S4 the input shaft had to be raised, which in turn raises the bell housing about 3/4" higher compared to the MD4 transaxle.  The other area to confirm is the clearance on the top of the transmission case.  Although the differential area on the S4 is the same height as the MD4, the S4 transmission area (the right 50% of the picture below) of the case is about 2" higher than the MD4 case.

The other major difference on the S4 is the shift linkage.  Since the S4 uses cables to shift, versus a shift shaft on the MD4, you should make sure you have the routing of the S4 shift cables planned out for your adaptation.

Other than the caveats noted above, an S4 transaxle will bolt up to the same mounts of the MD4 box. 

Click areas below for close-up view

1.67   1.59  1.56    1.50   1.44   1.37  

1.32   1.26   1.20   1.14    1.10   1.00  .096

Ring & Pinion choices:

 4.63  
 4.86  
     5.13 R *
5,43
     5.43 R *