1. I would not do so, it I were you.

2. I would not do so if I were you.
(Are both okay? Do we have to delete the 'comma' in #1?

3. He could come earlier.
4. He could come earlier if he were not busy.
(Does #3 mean #4?)

4. He could have come earlier.
5. He could have come earlier if he had made haste.
6. He could have come earlier if he had not been busy.
(Does #4 mean #5 or #6?)

In #1, I take it you mean "if" instead of "it" - right? The the comma is not needed.

All the others are correct and are similar in meanings, but not identical. The "if" clauses give reasons, but not all reasons should be assumed to complete the meanings of sentences 3 and 4 (the second 4!).

1. Both sentence structures are grammatically correct, but they convey slightly different nuances. In sentence #1, the comma indicates a pause or a slight change in tone, emphasizing the advice given, while sentence #2 is a continuous statement without any additional emphasis. So, whether to include the comma or not depends on the intended tone and emphasis of the sentence.

3. Sentence #3 stands on its own and simply states that someone has the ability to come earlier. It does not provide any specific conditions or reasons for why the person could come earlier.

4. Sentence #4 introduces a conditional clause that indicates a potential obstacle to the person coming earlier. It suggests that the person is currently busy, which is why they may not come earlier, but it leaves it open for the possibility if they were not busy. So, in this case, sentence #3 does not mean the same as sentence #4.

5. Sentence #5 introduces another conditional clause which specifies the reason for the person not having come earlier. The phrase "if he had made haste" suggests that the person did not come earlier because they did not hurry or move quickly enough.

6. Sentence #6 is similar to sentence #5 but provides a different reason for why the person did not come earlier. It suggests that the person did not come earlier because they were busy.

Therefore, sentence #4 does not mean the same as either sentence #5 or #6. Sentence #4 suggests that the person could have come earlier if they were not currently busy, while sentences #5 and #6 explain different reasons for why the person has not come earlier in the past.