Learn Dutch: Lijn vier valt vandaag uit. - Line four is cancelled today.

The verb being used here is the separable verb uitvallen. While it literally translates to "to fall out", in Dutch it is commonly used to mean "to be cancelled" when talking about public transport, classes, or scheduled events.

Why is uit placed at the very end of the sentence?

Because uitvallen is a separable verb. In a standard main clause like this, the main part of the verb (valt) takes the second position, and the prefix (uit) gets kicked to the very end of the sentence.

Is it possible to put the time at the end and say "Lijn vier valt uit vandaag"?

That sounds unnatural in Dutch. Time indicators like vandaag typically come before the separable prefix (uit), which strongly prefers to be the very last word in the clause.

Since this lesson is about time particles, how would I add al to say "Line four is already cancelled today"?

You would usually place al right after the time word, making it: Lijn vier valt vandaag al uit. The time particle stays comfortably in the middle of the sentence, safely before the separable prefix uit at the end.