There is a difference, but only a very slight one. The issue is really about where precisely present and past lie. You will see this in the following slightly altered version of your sentence.
We often talked about emigrating.
That, as I am sure you know, makes clear that that is over and done with and that we have not been talking of emigrating for some time. So natural continuations might be:
We often talked about emigrating, but the birth of our first grandchild changed all that.
We often talked about emigration, but now that Brexit has happened, we are going to do it.
The first of the two has the thought of emigrating ended by the birth of a grandchild. But at the time of the sentence, emigration is off the menu. The second of the two has the talk of emigrating ending and replaced by the definite decision to emigrate. In each case the event that brought about the change of mind is itself in the past and is no longer on the speaker's agenda.
Your two sentences, 3 and 4 have the following difference: the simple perfect covers the past in a way that stretches into the present and future. That is, talking about emigration has been going on, and has not (necessarily) stopped yet. The present is the present of habit, which implies that it is true of the past and may well continue into the future. If you used the continuous present, that would be different again.
We are talking of emigrating.
This implies nothing at all about the past. The couple may or may not have discussed emigration in the past. All this tells us is that they are considering seriously now.
Thank you for your question and welcome to ELU. I hope this is helpful.