Question

1. It must have been Chris.
2. It would have been Chris.
3. It could have been Chris.
4. It can have been Chrit.
-----------------------------
Thank you for your help.
Q1. #4 is incorrect. It's ungrammatical. Is that right?
Q2. Does # 2 mean a gueesing stronger than #3?

Answer 1

Q1 -- Yes, #4 is incorrect. You could use it in the negative (It can't have been Chris ... ) but not in the positive.

Q2 -- These don't mean the same thing.

<i>It would have been Chris if he had been called.</i>
This is not guessing; this is stating that Chris definitely would have been somewhere if he had been called to be there. (Context always helps!)

<i>It could have been Chris.</i>
This is guessing!