Explanation:

I can‘t work on this assignment anymore! I think I have bitten off more than I can chew.
bitten off more than I can chew: to bite off more than one can chew. If you say that someone has bitten off more than they can chew, you mean that they are trying to do something which is too difficult for them. He bought the old hotel but soon realized he had bitten off more than he could chew.
Hence, option 4 is the correct answer.