Let me start off by saying I have never played or tuned for GT4, but I have been reading this thread with interest, and given the fact that I did a lot of tuning for GT5P and my setups were quite popular, I thought my opinion may (or may not

) be of use here.
In my opinion: copying a setup, modifying one or two minor things and then passing it off as your own is lame, regardless of whether it's against any AUP.
Personally, I post my tunes for people to use in any way they see fit, meaning: use it, adapt it, tweak it, combine it with a tune from another garage, whatever you want to do with it, go ahead be my guest. As a matter of fact, I encourage that. I also encourage that you share the result of your work if you do, even if you only changed one minor thing. But it's an unwritten law that you mention credits where credits are due. And most people usually adhere to this 'law'. Of course, there's always the occasional exception to the rule.