"This is a standard 4" fit, and many soil pipes aren't. Mine appeared from the ground to be a standard 4" plastic pipe, but when up the ladder with the new vent it seems it is metal and nearer 4 1/8" or 105mm. Check if you need the alternative size!"
"I had a 50 year old metal basket style cap for my soil stack and it was rusted through. I bought this very cheap plastic cap. It fitted inside the existing pipe and I fixed it in place with a couple of self tapping screws. Cheap and cheerful fix."