Edit:  I Just re-read where you want just the 4mm part chamfered. There are multiple ways to do it but you can Split the body using the sketch, then Chamfer the edges you want.

Fusion 360 chamfertoolpath

In the attached screenshot, I have highlighted the 4 places (each time, a 4mm line) when I would lke to "chamfer" to create a notch so that I can deposit my electric toothbrush head.

With the help of a mysterious glowing ball of energy named Tri, Zelda is gifted the ability to travel into these rifts and heal them.

In my model you will see that the last step if to add .5mm filet on almost every edge (I feel I'm missing one, but can't find which one... 57 edges seems odd), si I guess I need to chamfer the section of line BEFORE I do the filet.

Chamfer fusion 360download

I feel like I am running around trying to accomplish something that must be quite simple: I have a line and I would like to chamfer part of it.

Chamfer fusion 360tutorial

@Anonymous, I have finally printed this part today. I'm happy with the result. You can see pictures of the printed part here.

El Tri has won fewer than half its games since Qatar, was eliminated in the first round of this summer’s Copa América and drew just 25,271 for last week’s Rose Bowl match against New Zealand, its smallest crowd for a U.S. friendly in nearly three years.

I am not familiar with the "patch" feature (but I'll look into it thanks to your video), but I tried to make 2 offset planes, then split the body with those planes and do the chamfer.

As you can see, I have even tried to add fake/construction lines so that I could select the part of line to chamfer, but it doesn't work. I tried to play with the split tool, but failed to properly understand how it works, apparently.

After struggling to select all the right (but not the wrong) edges to do a fillet all around, I got the idea to merge all bodies after doing the chamfer, and things look not so bad.