parallel sessions in zoom meetings (not BR rooms) - number of licenes

You could buy four licenses to accomplish this, yes.  By default, you wouldn't be able to manage them all under one email since you'd have to apply the licenses to four different accounts.  You'd have to log in under all four accounts to schedule and open these meetings.  Or, you could go into the back end of Zoom and give yourself scheduling privileges and alternative host rights for all four of these accounts to make it easier.

Zoom allows up to two simultaneous meetings per account.  So technically, you'd only need two licenses, and you would schedule two meetings under the two different accounts in this case.  However, when you do this, then a host needs to be present to start each of these meetings.  Zoom doesn't let you open a second meeting under the same account unless the meeting host is there to open the room for both of them, so you'd need to be logged in to start the first one, then assign someone else as the host, then leave and open the second room.  You'd have to repeat the same process for the other two rooms you scheduled under the second account.

Zoom also has a special license called "Concurrent License".  They have a few options for this, but one of them is a 4-simultaneous-meeting option.  It would allow you to schedule up to four concurrent meetings under one account.  Using this license would potentially help you save here.  I believe that this license is only available by contacting your account rep, however.  As far as I know, the Concurrent License can't be purchased from the regular Zoom pricing website.  So, if you're with an organization and have an account representative, you can ask them about a 4-participant Concurrent License.