I've been using KoneDriver for a while now, but a day came when I had to perform a factory reset on my computer (due to codec issues unrelated to hacks). After the reset, I tried using the same KoneDriver hack I had been using before, and it didn't work. I assumed that it was because I had reset my computer and some setting must be preventing it from working. However, it was during that time period that Infinity Ward stopped supporting PB (or PB stopped supporting COD4, whichever one) and therefore, PB was un-updateable for COD4. Well I found a fix for that issue, and THEN I tried using the same KoneDriver hack, and it worked.
This might be 1 issue preventing silent aim from working. If you would like to update your PB files, PM me because I can't post links to YT videos :/
There could be other reasons as to why silent aim does not work for you. You might have to update your drivers (because drivers are involved), you may also have to disable Windows DEP (especially if you are on Windows 8) which could be preventing the hotkeys from doing their job.
If you have any other suspicions as to why silent aim does not work, please post them here so others may try them. Thank you