I mean, if Nappa is only 4k, then there would be no reason for Goku to believe the mouth blast would've fucked him up had he not stopped it. Most people that have him that low don't believe the Saiyans can amplify their power.
Earlier, Goku thought Nappa's blast was so weak that all he needed was a kiai. Either Nappa got a lot stronger when he calmed down, or his mouth blast was obviously an amplified attack that went beyond his normal power. There's no other way around it. And Goku looked right at Nappa before he opened his mouth, so he wasn't off-guard.
Anyway, Krillin's quote tells me 5k Goku is still less than Nappa. Vegeta doesn't even think Goku can handle Nappa when he first arrives and makes a joke about it. He seemed to be more on edge when Goku's power was slowly rising, but Nappa was only outclassed after Goku powered-up.