What has Tybalt done in the opening of the scene?

In Scene Four of Act 1, he has attended the party at Capulet's place, just offstage. In Scene Four of Act 2, he has sent a letter to Romeo's house. In Scene Four of Acts 3 and 4 he has managed to get himself killed sometime earlier (actually in Scene 1 of Act 3). There is no Act 5 Scene 4.

Please specify both Act and Scene. To give the scene number without saying which act is like saying "He lives at number 68" without saying which street--absolutely useless.