I was pretty sick the week before Christmas. I took 4 independent tests in a 7 day span (2 rapid and 2 PCR), all of which were negative. I was diagnosed with pneumonia after taking x rays and had a lot of trouble breathing, but I also had the "typical" symptoms of COVID 19, except loss of taste / smell. I recovered in 1 week or so. Yesterday I took an antibodies test and it showed I had IgG positive antibodies. Is it possible that I had the virus, even after the 4 negative tests?

You likely had the virus at some point. Average guesstimate is that IgG antibodies will show up later into the infection, 7-10 days depending on the person. These are the longer-term antibodies, so you may have had it then, or a low symptom case in November or July or whenever.

What were your symptoms when you were sick? Were you coughing anything up?

Started with low grade fever (99-100), body chills that caused uncontrollable shaking and body aches. A few days later came the pneumonia type symptoms with shortness of breath and really struggled to pull in air, as well as coughing, but not much discharge from the lungs. Went on Doxycycline and Augmentin, plus prednisone. Most symptoms faded by day 3, except shortness of breath, which lasted almost 2 weeks.

Cactus Jack has it correct. The IgG's with COVID-19 currently seem to last at least 8 months if not longer. So you may have had it recently with false negatives or you had it sometime in the last 9ish months.

Thanks guys. Quick follow up question. Should I take vaccine, assuming the December illness was COVID? Or I had it in the not to distant past?

Yeah of course. Always take the vaccine if you can. Just adds to your immunity.