Писать подобные открытки другу, используя проблемы ниже i.ve been in for i tgought it would be it is and i enjoy the of the place you can,t imagine how it is

 read a thread about that issue, but I didn't find out the right answer.

What is the exact difference between:
1) I've been to London.
2) I've been in London.
3) Have you been to London?
4) Have you been in London?

Thank you