Who's to say that the Ellis-Founder from Star Trek: Voyager, Spirit Walk wasn't one of the four? The timelines don't match up exactly, but some creative storytelling could get around that. I don't think the timelines match up for the Ellis-Changeling having the morphogenic disease anyhow, 7 years before Spirit Walk (2378) would be 2371 and Homefront, the time of Odo's infection was in 2372.
Also, the Layton-Changeling from Homefront could have been infected upon briefly linking with Odo (while not a full on link, their forms did merge for a short time) and it could have later linked with its fellows on Earth before returning to the Great Link for new instructions.