The reasons why A4 paper is generally not allowed to be brought to concerts may be due to the following reasons:

1. Safety issues: Concerts are crowded and if someone carries A4 paper or other flammable Objects may cause fire and other safety issues.

2 Venue restrictions: The concert space is limited. If someone brings a large amount of A4 paper, it may occupy too much venue space and affect the viewing experience of other audiences.

3. Hygiene issues: The concert site is crowded with people. If someone brings A4 paper and other items, it may pollute the site environment and affect the health of other audiences.

A concert refers to a live performance by one or more singers or music groups in a concert or other performance form. Concerts usually include singing the singer's representative repertoire, as well as interactive sessions with the audience, such as Q&A, autograph sessions, etc.