Originally Posted by Todouble
That looks like an additional outlet to attach a flex duct and feed a supply register with. The furnaces have a bunch but not all units use them all so they use blank off plates for unused ones. That need to go back on.
Todouble is correct but before you seal it up, make sure that is not used to heat your under belly for you water pipes and tanks. Also I would check and see how many ducts you have connected. If you can not vent the correct CFM you are going to have a problem with the limit it not as easy as just taping it closed. I would do a little more research on it first. I did not see a duct on the other 4" outlet........
infact I do not see any ducts coming off the plenum. What unit do you have and what model furnace. How many registers do you have in the floor or low wall?