4 All the concert tickets ..... (already/sell out).

Passive voice explanation:

• The passive voice is used to show what happens to the subject.
• It is formed with the verb 'to be' (in the correct tense) + the past participle of the main verb.
• The adverb 'already' usually goes between the auxiliary verb and the past participle.

Answer:

4 All the concert tickets have already been sold out.