DS 14 had swollen knee twice before I googled and asked for the test. They kept wanting to send him to the orthopedist. And yes, he did flare yesterday horribly and it was a full moon - full of energy today though.
I know some 4 year olds wouldn't be able to answer accurately or truthfully but if your child generally does, you might try asking if they remember falling or twisting or when it started hurting. DS always swore he never injured his knee in any way or took any hits from lacrosse etc.
Google images of swollen knees and lyme and compare. I noticed that DS's knee swelling looked identical to photos after we knew what was going on. The swelling was more on the top outside of the knee and down along the side, mainly in front. Definitely take some pictures.
Also DS's swelling lasted for weeks so if the knee doesn't look better in a few days I would ask for the test. DS did get a positive test after the second knee flare.
Good luck with your own treatments and in figuring out your DK's knee situation.
Parent of 14 yo DS diagnosed with + Lyme test in November 2015, being treated for co-infections of mycoplasma (strongly +) and babesia (clinical diagnosis), mainly with antibiotics. Supplementing with Vit D, Vit C, multvitamin, milk thistle, Burbur and Pinella so far.
Website I started: ParentsofLymeKids.com