I am sticking my neck out here, and going to publish a tip from a reliable source. I am told the Glock 42 is a ultra compact single stack .380 ACP (as I predicted two days ago) and the Glock 41 has the same frame but is chambered in 9mm, also single stack. I am told they are about the size of the SIG P938. The Glock 40 is apparently a unique pistol and will be on display at SHOT Show, but I was not told what it is.
Its possible that my source was mistaken … in which case I will look like an idiot in the New Year!