This is a dimensionally accurate d20 with faces ranging from 1-4. Use 70-80% infill for an approximate d20 weight.

I saw a Kickstarter a while back for a 20 sided d4 (d420), I thought the idea was genius, but I wasn't able to back the project, nor could I find anything like it for sale. So I booted up good ol' Fusion 360 and got to work. The process for correctly labeling the dice's faces was a little trickier than I though. The dice number map is below.

A d20 is laid out so that it's result is random. To ensure randomness like numbers are placed further away from each other, for example the 2 face borders the 20 face and the 1 face borders the 19 face, which are on opposite sides of the die. Therefore, a good 20 sided d4 should be laid out as shown above.

Tips:

This model is a pretty easy print, just be sure to clean up the support material so it doesn't interfere with the dice's performance. 

As stated above use an infill of 70-80% for approximate d20 weight.