I heard they started/have started a new business.

[ROFTOK]

I can't put my finger on the difference between them. Can you point out it?

1 I heard they have started a new business.
2 I heard they started a new business.

3 I have heard they have started a new business.
4 I have heard they started a new business.

5 I hear they have started a new business.
6 I hear they started a new business.

 

[5jj]

The tenses and aspects are used in the ways they are normally used.

 

[ROFTOK]

I suppose they do. But what's the difference?
1 I heard they have started a new business. - 3 I have heard they have started a new business.
2 I heard they started a new business. - 4 I have heard they started a new business.

It seems to me that in reality 1 2 3 4 mean almost the same