I agree with Glenn. I think it would help if you thought about where the speaker is. In 1 & 3, the speaker has not left for Tibet while in 2 & 4, s/he is already in China.
Then, why are 2 & 4 in the past tense? you might ask...
2 & 4 are not in the past tense actually but rather in the future perfect. As Japanese does not have a different way to express that (as in "will have----" in English), it looks exactly the same as the past tense.
I can't be 100% sure though as I am no expert when it comes to these things.