Swelling 4 Years After Rhinoplasty - How Can I Determine What's Causing It?

What you describe sounds like you "feel" it is swollen. That's different than it is swollen and changes shape. Sometimes patients have strange feelings from there nose after surgery. These paresthesias usually last only a few months but may last longer. There is no treatment for this.
If there is actually swelling that recurs you should see a surgeon to make sure you don't have an infection or a cyst. We see patients routinely who have had 4 or more rhinoplasties who actually have thin noses and no swelling and no lymph blockage.

Swelling, though a main component of a nose job, should generally
completely subside within the first year or two of a rhinoplasty
procedure. Swelling that persists 4 years later is relatively rare and
is an indication that you should get a medical professional to examine
your condition. The cause of your swelling can be a multitude of
reasons, ranging from possible impact from your particular lifestyle,
complications of surgery, or a persistent infection. My best advice
would be to either contact the original surgeon who conducted the
surgery, or to seek the expertise of a new surgeon altogether.

Swelling is normal after rhinoplasty. In a primary rhinoplasty, most of the significant swelling subsides in the first 3-6 months, but can take up to one year or longer if the patient is a male or has thick skin. For secondary rhinoplasty, it take 1 ½ - 2 years and can be compounded if they’ve had other procedures as well especially if they’ve had grafts done along with the procedure. If abnormal swelling occurs several years after rhinoplasty, it can be due to an infection. However, this is very uncommon.