It sounds to me like some “bad bugs” (ambient bacteria or yeast cells) got into your wine. After four years of aging, having a batch infected due to a bad airlock must be heartbreaking I’m sure. Depending on what the organisms are and how long the wine was exposed, it’s quite possible that the wine might be all right to drink. If the wine still tastes and smells fine, you could take a chance and bump the free SO2 levels up to around 35, do a sterile (0.45 micron nominal pore size) filtration to exclude what bad guys you can and take it to bottle. You’re looking for classic oxidative spoilage cues like aromas of nail polish, nail polish remover and vinegar. If you don’t sense any of these indicators of microbial spoilage, and don’t perceive any spritziness or dissolved carbon dioxide gas in the mouth, it’s always worth a “Hail Mary” salvage attempt at bottling. As “no human pathogen can survive in wine” (to quote one of my UC-Davis microbiology professors) because of wine’s alcohol level and naturally low
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