1. turn Airplane mode ON
2. turn phone OFF, wait 10 minutes.
3. power up phone, turn airplane mode OFF
Luckily it worked on the 1st day, but it broke on the second day. Then I restored the 6.1.3 firmware and saw WiFi is back. If I synced lots of emails with a weak WiFi connection, the WiFi function crashed again. It should be a 6.1.3 firmware problem and not related to a hardware failure.
I decided to try a hardware reset (push Home and Power Button together) and restored the firmware again. Let's see what is happening.
Updated: after one more reset of the networking setting, it seems the WiFi has been OK now.
Updated again (7/9/13): it seems the WiFi chip was broken according to an Apple store staff. Then I got a new 4S from Apple.