Get the .tar.gz 4191 driver (versions are important here).
Decompress it somewhere.
Change to the new NVIDIA_kernel-* directory.
Do a patch -Np1 --dry-run </path/to/file.diff, and make sure you don't get any "x of y hunks failed" messages. If you don't, then you can do the same command without the --dry-run, to actually apply the patch.
If you do, then make sure the p option's argument is correct (almost all the time, it should be 1, though), and make sure you're in the NVIDIA_kernel directory. Also make sure that (in general) the patch you're applying was meant for the version of whatever that you have -- a patch against kernel 2.4.18, for example, will probably not apply to a 2.4.20 source tree right away. You can do it, but it takes a lot of work.