Reddit reviews uxcell 7 Pcs Air Pneumatic 6mm to 4mm Straight Push in Connectors Quick Fittings
We found 1 Reddit comments about uxcell 7 Pcs Air Pneumatic 6mm to 4mm Straight Push in Connectors Quick Fittings. Here are the top ones, ranked by their Reddit score.
I've had no luck finding PTFE tube aside from the common sizes!
However, I have found press fit connectors that let you step down from a 6mm OD tube to a 4mm OD.
You could likely use a couple to step between the two sizes of tubing. I don't know that this would work for your purposes, but it may be worth a try!
Here are some on Amazon
You can also find them on ebay for under $1 (USD) a piece.
Try searching for "4mm 6mm pneumatic push fitting."