1. David works hard, as do my brothers.
2. David works hard, as my brothers do.
3a. David works hard, as my brothers did.
3b. David works hard, as my brothers have done.
4. David works hard, and so do my brothers.
5. David works hard, so do my brothers.

Question:
Which is/are grammatically correct?
(Note: Sorry. I immediately removed it (my first sending this question) due to finding some errors in it.Now I have sent it again by changing it)

Question

1.   David works hard, as do my brothers.
2.   David works hard, as my brothers do.
3a.  David works hard, as my brothers did.
3b. David works hard, as my brothers have done.
4.  David works hard, and so do my brothers.
5.   David works hard, so do my brothers.

Question: 
Which is/are grammatically correct?
(Note: Sorry. I immediately removed it (my first sending this question) due to finding some errors in it.Now I have sent it again by changing it)

Answer

#4 is the best one. It's the only one that sounds natural and doesn't have any grammar issues.

You need to either add a conjunction to #5 or make it into two sentences.

The others with "as" are all okay grammatically, but they all also sound very formal and old-fashioned.