If by Guitar Hero 4 you mean world tour the answer is yes.
go into the xbox live marketplace and look around.
you can download songs online and insert it to your guitar hero game. search how to download songs to guitar hero two.
no.. not at this time
You can't download songs.
No
nope
No
you cant
No. How do you expect Guitar Hero to make any money off of download-able songs if people can just play their own?
You can't.
Illegally, yes
No, Guitar Hero 3 has no DLC, if you wanted to put new songs on it you'd have to hack it somehow. The new Guitar Hero: World Tour will have DLC though, you'll be able to download songs onto and play songs from your SD cards (they will probably let you download all the songs from the previous games). If you want more info on Guitar Hero: World Tour for Wii check out the Related Linksbelow.
No, you can't. :(