I am about to do this upgrade. Even if you can't get enough life from your SD4 to print all the parts, there are more than enough users on here more than willing to ship you the said printed parts. And then, you can always print backup carriages in extreme circumstances!

As far as the X motor (Nema 14) goes, http://www.thingiverse.com/thing:128526. If you did this, just buy a power supply and a 3-pin splitter, some 40mm fans, and some 3-pin extensions just in case. Again...if your carriage is melting, it is another reason to upgrade to an open sourced design.

And yes...the E3D is the end all be all in my book!
Hope this helps.

I just finished fixing my SD4 again, and as I was printing a replacement part (cosmetic) the X axis motor bracket started to melt.  It is currently still intact but the motor is loose and the belt has super low tension.  I was wondering if anyone has any sort of solid model or .stl of the left X axis motor mount so I don't have to shell out $100... I could probably get a few more prints out of this thing before it droops down like molasses.

That being said, I have so far had the following system failures on my SD4:

-Supplied X motor burned out -- back of envelope calculations showed that the supplied motor at the recommended voltage draws more current than a NEMA 14 can take.  I replaced it with a NEMA 17 at a 45 degree angle so it fit, which never got hot and worked very well.

-Feed gear "perma-clog" -- When I had an extruder clog the extruder kept feeding and dug into the filament and bunged up the teeth on the feed gear.  I was able to clean it out with acetone and mechanically  but it just never really fed properly again.  I ended up machining a new hobbed feed gear with a pitch of 16tpi -- it grabs the plastic and doesnt let go!

-Extruder "pistoned" -- Liquid plastic pushed the PEEK up into the feed gear and stalled it over and over.  I had to replace it with an E3D as it does not use PEEK.  To supplement this, I machined aluminum extruder parts that work beautifully.  The E3D is amazing, i would highly recommend it.

-Belt tracking on Y axis got all messed up.  I don't know how it started, maybe it was a vibration thing or something, but it took awhile and what seemed like endless watching of the stupid how-to video to fix it.

-Motherboard died.  Just died.  Couldn't connect to any computer, no indication that it was alive, after weeks of troubleshooting I ordered a RUMBA.  The RUMBA is awesome, I would recommend it.  It took a few weeks to figure out the setup, but I got it working.

-X axis left side melting.  X axis motor gets warm but still within spec.  I would suspect creep deformation from a warm environment. 

So did I just get the worst SD4 ever made??  Has anyone else had this many problems??  I feel like I got fleeced and now I'm even thinking of getting another 3d printer (which of course I'll have to save up for...)  I will admit that I use it a lot -- every night -- but I would expect a consumer level 3d printer to last for awhile...