I used to do Oracle Forms which seems to be considered 4GL. It was a great tool. I loved how you brought in your data access all via these things called Data Blocks. I think this data-centric nature distinguished it from the Borlands and Microsofts of the same time. That said it relied very very heavily on an Oracle database although you could get data drivers to talk to other sources.
The visual aspects of the system were great but you asked about language. I recall it had triggers like When-Create-Record and When-Remove-Record. You could navigate records, show and hide form elements, etc. But it all required the RAD tool.
I loved it. There were some things to dislike but you could be very productive with database heavy applications and the way the tool organized things made it generally easy to understand an app and thus maintain it. The biggest issues were in the visual part which maybe speaks less to your 4GL question.