@john143
"Can I add a fourth custom field to invoices in QB Essentials? There doesn't appear to be a way to do so. Is there a max allowed for QB Essentials subscriptions?"
Currently, there is no way to add fourth custom fields on sales forms in Essentials or Plus or Simple Start.
In the Advanced plan only, they have enhanced custom fields but showing to customers on Invoice form is still limited to 3 fields but more fields for internal. Refer to this article for more info:
https://quickbooks.intuit.com/learn-support/en-us/sales-orders/create-add-and-edit-custom-fields-in-...
Hope this helps!