I have an extra 4g pinfire shotgun shell for sale (it's not cheap), as well as an 8g centefire; both by Gevelot.
Here is an image of the 4g shell and box they came from. And then an image of the 4g pinfire, 8g centerfire, next to a typical sized 12g centerfire.
Also, that was my gunbroker auction posted a couple pages back, so as seen there I also have many other sizes of pinfire shotshells.
Also, I do not have one, but they also made centerfire and pinfire shotshells in about 32mm (for comparison the 4g is about 28mm) . And here is a pretty good list of the different shotshell sizes, from a friend of mine's book he is working on.