depending on how you did with the first three fill coats, you could get by with as few as no skim coats....but if done with good technique, not more than one.

the clc site has a nice little piece on filling the weave.

https://www.clcboats.com/shoptips/epoxy_and_fiberglass/fiberglass_weave_epoxy.html

so to get more directly to your question, the skim coat is, hopefully,  the last coat of epoxy to fill any final remaining scratches, pin holes or other minor, impefections after the fill coats have been sanded back with a ros sander and/or fairing board to get a smooth surface.   it is usally a very thin coat applied with a only roler and then gone over with a firm plastic spreader/squeegee to only fill those remaining imperfections.  my skim coat on a 17 foot boat hull may be as little as four pumps of resin and corresponding hardner.

if after your sanding your fillcoats, you don't have any imperfections you are concerned about, you don't even need a fill coat.

i hope this helps.

h

   Thanks so much, this is very helpful.  The article you sent is excellent, I wish the instructions had included the same content or at least a link.   Thanks!