Dear N:

You wrote:
> Consider the following conversation:
> -You haven’t really tried to go in. What have you done to go in?
>
> 1-I have knocked on this door and I still am.
> 2-I have knocked on this door for ten minutes and still am.
> 3-I have knocked on this door for ten minutes and still am knocking on it.
> 4-I have been knocking on this door and still am.
>
> Which of the sentences 1 to 4 is acceptable?
>
In terms of style only #4 would be considered standard because only #4 has a parallel verb construction. There does not appear to be any reason to change the tense.

#1-3 might occasionally be heard in colloquial conversation but really do not make much sense. “Have knocked” implies a single action in the past, so if you have already knocked, the implication is that you were done. The progressive tense more precisely shows that your action of knocking was continuous even up to the present.