I've 4 entities on Quickbooks and I used them all at the same time for company operations and transactions, and it's not possible to open different entities in tabs.

You need to utilize the private/incognito browser to do so.

QBO account 1 = Chrome - primary

QBO account 2 = Chrome - incognito

QBO account 3 = Edge - primary

QBO account 4 = Edge - private

You can use other browsers if required.