Background
The M4 rail wallet is the latest small form factor card carrier style wallet from Dango.  It manages to have a remarkably slim profile, while still being able to hold a decent number of cards as well as some cash.  Unlike the other rail wallets, rather than a metal prong which over time wears out your cards, this one uses a replaceable plastic prong.  While they work great, it isn't for long as they are made with the most brittle plastic known to man. 

Material
I modeled these directly off of the Dango ones with the ever so slightest of changes, and when printed in PETG they are significantly more resilient while retaining the same strong hold on your cards.  You realistically have to use PETG as it offers some flex and can handle repeated deflection; PLA is far too stiff.

Print Settings
Quality
Ironing: Top surfaces
Ironing Speed: 60mm/s
Ironing Flow:  30%

Speed
Initial layer: 20mm/s
Initial layer infill: 30mm/s

Setup and Use
You can print them as oriented with supports, or put them on the angled part pointing diagonally upward.  Laying flat with supports gives the strongest results I think.  Also, to ensure a clean release, I used PLA as the support interface material.

Final Remarks
The Dango M4 is one of my favorite wallets save for the rail prongs.  Using plastic is a great idea over the metal ones which mess up your cards, but the OEM prongs are quite fragile and I've already gone through 3 sets of them.  They're cheap enough at $2 per set (was $6ea, but likely discounted due to the frequency of failures), but I'd rather not have to keep replacing them and these have worked out great so far.

Photos
Red - Original broken prongs.
Blue - PETG replacements I printed.