CONCLUSION:  The murderer is No. 4 and there are three proofs:

1. The knife (a tool of murder that is next to the victim), is missing from the table.
2. His clothes are as if he was physically in contact with someone and is right next to the toilet door (although they are women’s toilets the rest of the customers do not have the ability to see who comes in).
3. Finally, the observers would see that behind the murderer’s ear, there is blood.