I have brewed a weizen beer on the 23rd of October this month. I have a chest freezer I have converted to a fermentation vessel using the ST1000 which works like a charm. I live in San Clemente, Ca. and the temperature in the garage currently is holding the fermentation right at about 18 C. I grew a vile of wyeast #3068 for 36 hours and pitched it, the fermentation went off around 7 hours later and was very aggressive. As of today the 27th I have seen no activity to speak of however in the air lock water there are still bubbles on the surface of the water so I am assuming it is still active. I want to make sure before I cold crash this beer, as I am going for a crystal weizen look and feel here that the fermentation is complete.
Is it possible the fermentation is completed after only 4 days?
You need to verify you’re at final gravity. Do this by taking 2-3 hydrometer readings, 2-3 days apart each. It’s the only way to know the yeast is finished.
To answer the question, yes, it is not only possible, but likely done. You still need to check and it wouldn’t hurt to let it work several days longer.
Hefeweizen yeast strains are notoriously fast. It’s most likely finished, that’s very normal. But, like the others suggested, take gravity readings a few days apart just to make sure. Then you’ll be ready to bottle or keg… or cold crash if you want kristall style.