Right here:
http://www.megagames.com/psx/psx_patches_g_2.shtml
Scroll down to Guitar Freaks, download the +4 Trainer and anti-mod patch, apply and burn. I've tested it: it works. Be sure to apply it to a .bin file, and go ahead and patch even if it says the file size is wrong.
Also, when you get to the "trainer" screen (Little thing that boots up and gives you extra options like "Start with all songs") Press X to launch the game in PAL (UK) mode, or press O to launch it in NTSC (US, Japan) mode. This little detail took me a bit to figure out.
Hey, who knew a first post could be so helpful? =P
Edit: Semi-helpful :/ I just noticed this was bumped from like 5 YEARS ago. But hey, I still think it's helpful...not a lot of detail was given about that particular patch, I had to figure it out on my own.