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 pH, the wine will be perfectly safe to drink. However, even if it does smell and taste fine today, be aware that the wine may not age as well or as long since it has been prematurely exposed to a higher than desirable level of oxygen.
Since you describe a film on the surface of the wine, you are probably dealing with some kind of microaerophilic organism like the kinds of yeast encouraged to grow on some kinds of Sherry while aging. These yeast cells actually consume alcohol, metabolizing it into acetaldehyde, and in turn imparting some of the oxidized, nutty and toasty flavors we associate with this kind of specialist “flor Sherry.” However, depending on what kind of wine you made, especially if it comes from fruit other than grapes, these characteristics may not be welcome. If you do in fact like the effect the film has on your wine, you might want to try leaving one carboy intact, just as an experiment.
If the wine does show signs of objectionable aromas and flavors, then there is really not much you can do. Sometimes adding excess sulfur dioxide, like bumping the FSO2 up to around 45 ppm (maintain at 25-28 ppm FSO2 thereafter) can bind up aldehydes and reduce some of the objectionable smell. It will still be necessary to filter the wine as you obviously have microbial visitors that can still consume oxygen and spoil the wine in the bottle.
You are correct that you should’ve transferred the wine out of the carboys, and out from under the airlocks long ago. Typically we rack the wine from a fermentation-lock container down to a barrel, or at least to a carboy with a “hard bung” airtight closure, after all primary and secondary fermentations are complete. For most wines, this occurs between 2-4 months of age. Four years is also a long time to age a wine before bottling. Most top-scoring red wines made in the U.S. are bottled after 18–24 months in barrel. Whatever you do or whenever you bottle, please protect future batches by getting it down into barrel, or at least into completely topped carboys sealed with something more solid than a topper that depends on an evaporating seal of water to protect it.
