Does this sound like a reasonable deal to make my brother?

He is having money problems and wants to pay off his car. I'll pay off the $11,500 he owes, if he agrees to pay me back at 4% interest whenever he can in the future. I'll write something up for him to sign. 4% is what I get from a CD or high-yield savings account, so I think that's fair.

Does this sound like a reasonable deal to make my brother?
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