I have GR4 now, but whenever I go to use it with my guitar, I get an unholy amount of feedback and background fuzz.
I'm not using any interface of any sort, just a line in.
Is that the problem? or could it be something else?
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It's most likely the fact that you don't have an interface but those amp sims tend to be noisy as well. Depending on how much gain you use, you're going to get a fizzy/hissy sound out of them no matter how much you play with the settings.

You can try putting the noise gate on and see if that works for you.
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You definitely want an interface, coming from experience here.

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You could ask the developer which interface would work best with your pirated new software...
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