Infinity Ward has shot down speculation regarding Modern Warfare 4, denying any contact with the purported source of the rumor. Word began spreading yesterday, after a report that claimed voice actor behind Captain Price had said he was signed on to the next project.
"Interesting news today, but it's not true," read a response from the official company Twitter account. "We've not talked with any voice actors, so all news is speculation."
The denial doesn't explicitly claim that Modern Warfare 4 isn't in development, mind you. Call of Duty is on an annual release schedule for the foreseeable future, so it's reasonable to assume we'll see a MW4 eventually.