Reverse Bowden for your MK4S - uses collet clips to secure both ends.

Have you ever put a full spool on your MK4/MK3.9 filament holder, and have it come unwound during a side to side print?  The left right motion of the print head yanks on the print spool and can make it over-run.  Even with brakes on your spool, it can still come to a big tangle and failed prints when you start out with a full print.

This is a complete mod - you print three different parts - add about a foot and a half of PTFE tube, and the tangled full spool problem is gone forever.

I got this idea from my Voron - which never ever tangles.  Why?  It uses a bowden tube setup to feed the print head.   By making a screw-in PTFE tube adapter for the Nextruder, and putting a PTFE tube adapter on the frame instead of a filament guide, the yank and tangle problem disappears.

Now, to be certain, this isn't the first cow out of that barn on Printables.  There's adapters that use Pneumatic couplers.  There's adapters that use PTFE tubes that are just stuck into a part that is a little tight.  This one is secure and held secure by collet clips - either the kind you can buy, or the kind you can print such as this nice collet.  

Now, just print these other two parts, one trimmed so going full height doesn't wonk you during calibration, and one that screws into the Nextruder filament hole.  Print two collets, one short frame adapter, and one nextruder adapter, and add a foot and a half or so of PTFE tube.  You'll never unwind a spool around your filament holder again. 

To print - Load these projects as "Open Project" in prusa slicer.  That saves you a lot of time fiddling with settings.  Also, when the file opens in the slicer, answer YES to the multipart file question.  Slice, print and go.  Just add PTFE tube.

About the files - the frame mount part is a remix of a part that I downloaded.  It was way too long and interfered with the spool, so I shortened it and added a "collet friendly" interface.

The Nextruder part is also a remix of a part I downloaded here.  But it was not collet friendly and required a separate screw-in piece.  Collets are the way to go.  The tube can be inserted and removed, no separate parts, and it's pretty much a standard.

Why didn't I call this a remix?  I made way too many changes and used several base models that I tried out and they just didn't work out so good.  This is now packaged in one listing - two parts and a two collets and a bit of tube.  It's a quality of life upgrade.  It's a little harder to load filament because you have to push it thru the tube.  But it's not much and it'll never tangle ever again.