It's very easy to be worried and confused about this disease in pets. When caught early, it is mostly treatable and a lot of dogs never even show symptoms. My beagle tested positive 6 months ago. Your vet will likely prescribe antibiotics as a precaution despite being asymptomatic. What I found to be helpful is doing a urinalysis to check for any protein in the urine that can arise from kidney problems - a "silent" killer of lyme disease. Again, when caught early, you can treat this... I am still learning about the process as well and have a check up apt for my dog tomorrow - I will let you know if I find anything else out.
Just want to emphasize that, as the vets tell me, testing positive for lyme only means your pet was exposed to a tick carrying the disease, it doesn't necessarily mean your pet is infected with it.
I recommend the Seresto collar for flea and ticks if you don't already use a preventative. My dog was on K9 Advantix II and he got bit anyway, just be cause of how rampant they are and how much we frequent the woods. I think the oil wore off quickly due to frequent swimming as well, which is why I love the collar right now.
Good luck, hang in there, and don't get too worried about this. 4 years old is young, I am betting your pet's immune system is pretty well in tact.