I've had a cold for 4 days and now my sinus cavities have a lot of pressure and I cannot breathe through my nose.

OK. That is fairly usual with a head cold. You can try an OTC decongestant to help relieve your symptoms until you can get to see your physician to determine exactly what ails you and if any other meds are in order. Feel better soon.
Not unusual. Please make an appointment with your family physician as there are prescription medications that are much better than OTC drugs that will relieve your congestion. I would also recommend using hot wet compresses to the face for 5 minutes 3 times daily & if you smoke STOP NOW if not forever.
Seasonal flu. is very common viral infection. Usually the condition resolves on its own with home care. Drink water and get a good rest. NSAIDs (e.g. Aleve, (naproxen) Advil) are also recommended every 6 hours. If concerned, see your PCP for examination.