I was talking to her about something else when she suddenly started to talk about what he had been up to these days.

I was talking to her about something else when she suddenly started to talk about what he was up to these days.

I was talking to her about something else when she suddenly started to talk about what he has been up to these days.

I was talking to her about something else when she suddenly started to talk about what he is up to these days.

Which of the above sentences grammatically correct? What's the difference in their meaning?