If it can do 4GHz at 1.55V, I'd say you're fine. If you're asking if I think you should buy a second one, that's up to you. Personally, with a result like yours I'd probably just run with it. If you're asking if I think you should exchange it for a different one, absolutely not. That would be unethical to exchange it because it won't overclock enough when it is functional at stock.