Since we're talking about Daima, what exactly does Goku mean here? Vegeta talks like Goku got SS4 before and was hiding it, and all that Goku says is that Buu was really tough so he trained a lot, and "wasn't sure if it'd work". So what does this mean? Did he actually get SS4 before and wasn't sure he could use it as a kid and needed Neva to give him that boost? He did wonder initially if he could go SS1... Or just sensed the form when training and couldn't access it until Neva gave him that push? Though Vegeta talking like Goku got the form before throws a wrench in this theory...

Maybe the Japanese dialogue is clearer? Sometimes subs are iffy.