Is it a good idea to reveal, in my query, that I’ve written four unpublished novels?

No, no, no, and no. The number of novels you’ve written are not important at this point. What is important is the fabulous-ness of the one you’re marketing. You can discuss other novels with the agent after you’ve agreed to work together. Sometimes, after the first novel is on the path to publication, past works can be resurrected, revised, and sold. Suggest, in your third paragraph (which is all about you) that you comment how you’ve been honing your craft and writing for xx years and participate in a critique groups, attend conferences etc. Describe the actions you’re taking to become a more skillful writer. That counts!