Heyho

Here I have a PTFE Tube Connector for the standard 4 mm ones. I use them at my XL to connect my Teflon tubes from the dryboxes to the ones sticking out of the first Filament sensor.

The features:

A segmented inner area for a good force distribution/ clamping strength without deforming the tube

100% Scalability: 
3 Prescaled Models: Soft Medium and Strong clamping force (1, 2 , 3 Dots)
If you only want to change the clamping pressure just scale the X/Y Axis! If you just want to change the length without changing the clamping force just scale the Z Axis.
This way you can also scale the Adapter fitting to other tube diameter.

Wide ends to increase first layer adhesion and a better grip if you want to push them on the tube with a higher clamping force.

Viewing window to check that the 2 hoses touch perfectly.
 

Assembling

To test the best fitting I would recommend to start with the strong one (I use that) or just print all 3 Versions first and then multiply the best fitting ones. Or Scale them to your needs!
 

You can add a little chamfer on each Tube end but more important is to cut the ends off 90° as good as possible. 
If they fit correctly there will be no gap between the 2 Ends.

Printing: I printed mine with PLA, 0,2mm layer Height & standard Infill. Seam set away from the window.

PS: I intentionally did not design a hard stop in the middle of the connector to make the transition between the tubes as smooth as possible.