What 4c process blend best represents a yellow highlighter?
I'm creating a letter and want to include a highlighted section, I'm trying to find the best CMYK blend that represents that when printed.
I would contend that this is difficult to answer definitively.
If you're talking a four-color process, a straight 100% 'Y' would probably be your best bet. However, highlighters tend to have a neon glow about them, which can't be achieved in CMYK. You'd need a special process color for that.
Get your hands on the highlighter you want to emulate, draw on paper, and use either a swatchbook or printed samples from your local printer to experiment. That's going to be better than anyone here giving you an answer.
If you want to include a highlighted section and you want realism though, the best way to go about that is to study what physical highlights look like in terms of the shape of the highlighted block. Computerized facsimiles look fake because they're perfect rectangles. If you give it a bit of a shake, and one edge is a bit rough, and the thickness isn't uniform, that might go a long way in making a highlight look real.