Moonskin93 provided a very detailed skin overlay for Sims 3, which outlined the rib cage and spine of a sim, along with defining the hips. Seeing as how the skin was taken down, I looked and saw that Moonskin93 is very generous and allows for their CC to be edited as needed.
The custom skintone Blood Drinkers by Moonskin93 seemed to have incorporated bits and pieces of the original 'skinny skin' so I took care and converted over the more defining bits of the skintone to Sims 4. What resulted is a quite subtle outlining of a sim's ribs, hips, and spine so that they appear quite frail, listed as a 'forehead wrinkle' skin detail.
The detail is very versatile and as far as I've tested it works great with Maxis-match skin tones.
In no way do I encourage eating disorders or am poking fun at weight. This is very much for storytelling purposes, and to add a touch of 'realism' to my frail sims.
Provided in the .package is a skin detail that can be used as a 'forehead wrinke' because I never use that detail anyways, haha. There are three flavors of the Skinny Skin detail. One is a more dramatic outline, the other is much more subtle, and the third one is the same as the second only it doesn't include the spinal outline or defined hips. I have only included comparisons of with the most dramatic overlay and without the overlay at all. The two other subtle ones are just much lighter.