I can't wait either! I have finished the third book in the series -- it is called The Man with the Silver Nose. It introduces new adventures for Colophon and her brother at The Tower of London, Windsor Castle and Westminster Cathedral.
The decision, however, to publish another volume in the Shakespeare Mysteries series is ultimately up to the publisher, Houghton Mifflin Harcourt. If you are inclined to do so, please email the publisher at email@example.com and let them know that you would like to see the series to continue. Books are published for readers, and publishers want and need to hear what readers think.
I do have another book coming out in 2017 — it is a mystery based on the artist Vincent Van Gogh. Keep an eye out for it — I’m sure you’ll like it.