A tough case: Even during an attack I would find a 4.2 Uric Acid too low to indicate gout, even given the typical fall as crystals form. Typically the first gout attacks subside untreated after 3 or 4 days.
If it IS gout, then allopurinol will clear your symptoms forever, as long as you keep taking it.
But if I were in your shoes, I would watchfully wait until you have a clearer picture and more certainty that it is gout. Taking an antibiotic for 10 days unnecessarily isn’t serious, but taking a drug for the rest of your life if you don’t need it is a horrific idea, as much as I like allopurinol.
You can get some more evidence by taking colchicine. If it works well that points to gout.
Get a couple more serum uric acid blood tests.
Alas for most of us the confirmatory event is the attack from Hell and very high SUA levels…but then we are sure. In your situation, if you start on a life of allopurinol now, you will always wonder “what if it isn’t gout?” Until you see a SUA of at least over 7.0 don’t begin treatment unless they can isolate crystals from some joint fluid.