Ear infection is back for the 4th time this year — is there an underlying cause?

Four infections per year is above the threshold where you should be looking for an underlying cause rather than treating each episode in isolation. The most common culprits: environmental or food allergies (even grain-inclusive foods can trigger — the protein source is often the issue), hypothyroidism, or anatomical factors in the ear canal. I'd ask for a referral to a veterinary dermatologist, or at minimum push for an allergy workup and a culture/sensitivity test on the next infection to make sure Otomax is actually hitting the right organisms. Monthly bathing is fine but dry the ears immediately and completely — even slight moisture in floppy-eared breeds perpetuates this cycle.

The protein source allergy connection is real and under-diagnosed. Chicken is the most common culprit — if he's on a chicken-based food, a 12-week hydrolyzed protein or novel protein food trial can be revealing. It's annoying to do but often the answer for dogs like this.