If you followed the directions in the FAQ exactly then the best bet is not enough heat. Try moving it to a warmer location - near a sunny window. Next best guess is too much water with not enough pulp from the tomato. Was the tomato really really ripe (overripe) almost mushy - any you squeezed in pulp along with the seeds?

Dave

Four days is too long, unless the temperature is too cool, which is not likely at this point in the season.

I agree with Digdirt's assessment. If there is too much water, fermentation may still take place - but there would perhaps be no visible indication, and sprouting could occur. Personally, I disagree with the FAQ's recommendation to add water to the mix. In cases where a particularly dry tomato had too little juice, I have used the strained juice from a wetter tomato, to provide the volume necessary.

Inspect the seeds, to see if the gel sacs have ruptured, in which case the fermentation is done. If you find signs of sprouting, and have enough tomatoes for another attempt, it would be best to start over.

it's also possible your house is too clean.

Wherever fruits / produce have been decaying before (as in always the case with those of us who bring in ALL the tomatoes from the patch for salvage processing)... and wherever yeast breads have been baked for years... there are already a lot of fermentation/spoilage micro-organisms present.

They're on the walls, in the fibers of the curtains, etc. If you live in a fairly new home and haven't had unsound produce around the kitchen, you may have very little of the needed micro-organisms.

In the outside world, fruits and vegetables are "innoculated" to spoil naturally (that "bloom" on grapes is wild yeast just waiting to make wine"). Try the previous suggestions to reduce the amount of water to almost all tomato pulp, and then squeeze in a few drops of juice from a rotting tomato... the kind we mostly throw over the fence... that should start your fermentation real quick.

In years past, I did only a few seeds for myself, and I kept them in sherry glasses on the kitchen shelf. Now that I'm doing bigger batches, they're in glass jars in an outbuilding and checked daily.

Also I only rinse my jars after use, making sure all seeds of previous batch are out. I do not wash them as one would dishes to eat from.