I can't answer your question about the 4140 vs 4130 skin wedges, but I still might be able to add to the conversation. The only skin spoons I have ever owned/used are the ST991A PTI model, used by Boeing on their airplane assembly lines. I'm sure Boeing would use something different if there was a brand that made a better quality tool, because Boeing doesn't skimp when it comes to tools.
I purchased the ones I have from Ebay, where they usually come up for sale in groupings of 2-6 a few times a month. Sellers almost always call the Boeing version a "skin spoon" instead of a "skin wedge" in the listing title, so use "skin spoon" in your searches if you want to find listings for them more reliably.
I'm not sure if the ones Boeing uses can be purchased new from an online source or not. They are made out of heat treated steel plate, but I don't know what type of steel they use. The long handle is 12 gauge thickness, and the curved prying section is noticeably thinner and it tapers down to a nice sharp pointed tip. With the ones I have, I could probably cut into my skin if I swiped the pry tip across my arm fast.
If I do pry on one too hard, the tip will bend instead of breaking (which I prefer), and the tips can be bent back and straightened up quite easily and they will continue working almost like new. Two of mine are used as dirty work or sacrificial tools for tougher jobs, and they have tips that have been straightened/repaired multiple times each.
After looking at the skin wedges that are marked 4130 or 4140, I have a couple thoughts. The pry end is curved instead of almost perfectly straight like mine. With mine I can push the wedge slowly between two parts I'm trying to separate (like to help release a stuck gasket for instance) and the parts will separate without even needing to pry at all. If the tip is curved, it becomes harder to use that technique because the tip will always start wanting to dig into the top part or the bottom part as you push the pry tip further in and the curve starts to become involved. On the other hand, a curved pry tip that's a little bit thicker and stronger might be better for jobs like prying apart vehicle interior panels. The 4130/4140 skin wedges appear to also become thicker faster as you travel away from the pry tip. Whether that is good or bad will depend on the application you want the skin wedges for. For people who will use this sort of tool a lot, it might be best to have a couple of the thin straight tipped Boeing style spoons and a couple of the thicker 4130/4140 style curved tipped wedges.
Lastly, the skin wedges on Amazon (linked to by lardy1 in the next post) show the tip fitting into a 23 gauge sheet metal gage. My skin spoon tips will fit into the 26 gauge slot of my Starrett 283 sheet metal gage, so it appears that the pry tips are thinner on the Boeing style skin spoons.
en.m.wikipedia.org
