Hello Miss Snark,
The more research I have done on publishers and agents, the more anxious I have become to insure that I don't waste their time because of ignorance. There really is a great deal to learn about the submission process!
I am preparing to submit queries for my first work, a prequel to a four book, fantasy series. I decided to submit the prequel first because I read that agents/publishers are much more willing to take a chance on an unpublished author with a standalone work than on one with a series.
Have I made a wise choice by submitting a standalone work first? Should I spend a paragraph talking about the series that follows or should I even mention it at all?
You're worried about the wrong thing in the first paragraph. It's not a waste of my time if you query like a nitwit, its a waste of YOURS and your money. Despite all our self serving statements to the contrary, agents are not deities and our time isn't worth any more (or less!) than yours. You want to get this right so you can present your book in the best possible light to an agent. You want to get this right for YOU not for me. Miss Snark may be bald but she's not Yul Brynner and you are not Deborah Kerr.
The people that waste my time are the ones that never think they are; the ones with no clue that the submission process IS something to be learned.
That said, query on the prequel as a standalone. You can mention you envision a series but don't spend any of your limited word allotment in a cover letter saying much more than that. It won't be important unless the first book gets launched.