So, let's say an alien species has 4 lungs: a pair of smaller lungs for passive breathing and a larger set for conscious breathing and vocalization. Airways are separate from the digestive system. would it be possible for a sort of muscular valve to block airflow evolve into a sort of tongue?
If you mean an organ used to produce speech, yes it can. It happened with birds in our own world.
Birds vocalize not with their tongues, but with an organ inside their chest called the syrinx.
The syrinx (Greek σύριγξ for pan pipes) is the vocal organ of birds. Located at the base of a bird's trachea, it produces sounds without the vocal folds of mammals. The sound is produced by vibrations of some or all of the membrana tympaniformis (the walls of the syrinx) and the pessulus, caused by air flowing through the syrinx. This sets up a self-oscillating system that modulates the airflow creating the sound. The muscles modulate the sound shape by changing the tension of the membranes and the bronchial openings. The syrinx enables some species of birds (such as parrots, crows, and mynas) to mimic human speech. Unlike the larynx in mammals, the syrinx is located where the trachea forks into the lungs. Thus, lateralization is possible, with muscles on the left and right branch modulating vibrations independently so that some songbirds can produce more than one sound at a time.
We vocalize using vocal folds at the throat and the tongue at the mouth to make different sounds. Birds can do it using just the syrinx. Your alien could have a syrinx too.