Hi Jinyoung, while the acidic aroma of your starter isn't necessarily a bad sign, if your starter is doubling in 4 hours then it sounds like either you live in a very warm climate, or are using very warm water to feed your starter. Ordinarily at about 70ºF/21ºC our sourdough starter will double in about 8 hours. Since you're starter is experiencing more hours after its peak in which to fall and become more acidic, you're building up more acidity. Ideally you want to feed your starter again when it's at its peak of rising, or just beginning to fall. While it's not always possible to catch it at just the right moment, you can adjust the rate of fermentation by using cooler water, and perhaps refrigerating your starter when it starts to fall and isn't due for another feeding for several hours. You can even reduce the percentage of starter as compared to flour and water fed, to slow things down a bit (you might try something like 1:2:2). The goal is to find a feeding routine that keeps your starter nice and healthy and active (with a pleasant aroma) and doesn't interfere with your own schedule and responsibilities. I hope this helps! 

Barb

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