We want to create a shared mailbox like support@ for four people. Do we need to buy a license for it?

No license is needed for a shared mailbox up to 50GB. Each of the four people uses their own license to open it.

You need a license when:
• the shared mailbox must grow past 50GB (an Exchange Online Plan 2 license raises the limit),
• you turn on an archive for it, or put it on litigation hold.

Create it in the Exchange admin center under Recipients > Mailboxes > Add a shared mailbox. Do not sign in to it directly. Add the users as members and they will see it in Outlook after a short time.

One tip, remove the password sign in for the shared mailbox account after creating it, so nobody uses it as a normal mailbox.

Created it with no license. Team can see it in Outlook. Thanks.