As stated in title, i have MB4.1 and I know x3100/gma950's are a pain in the A, as there is no drivers. I didn't find any report on will it run with 10.8, i know for sure that 10.9+ isn't worth it as there is no accelaration.
I've never run 10.8 on any machine so I don't know if it's even usable. I know that 10.7.x isn't. So much has changed that Safari on Lion isn't capable of decent web browsing and it's probably as bad with any old third-party browser that might be installable.
To answer your question…from the info at osxhackers.net it would seem the graphics will work in 10.8.x.
Install OS X Mountain Lion, Mavericks, Yosemite, El Capitan, or macOS Sierra on your Unsupported Mac.
osxhackers.net
I am in the process of getting El Capitan on one of these MacBook 4,1 models. I've got it running on an SSD in there but I'm waiting for the RAM upgrade and a new battery (this one is completely dead). The fans ramp up on the push of the power button but I'm hoping a working battery will solve that.
I tried to get ML on it, just to try, but couldn't. After all this I think I'm going to put a Linux distro on it. It's for a kid and everything she needs it for is web-centric…and the OS I chose is up-to-date on web browser support. The graphics are supported and the OS is light enough to display full-screen HD streams most of the time.