ets Jan 8 2013,rst cause:4, boot mode: (3,7)
Further dig into reset info to get
reset reason : 1, wdt reset
epc1=0x40000f68, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000000, depc=0x00000000
Espressif documents says "Please do NOT disable the hardware interrupt for a long time, it should NOT be longer than 50us" or it migt bring about a hardware wdt reset.
Howver, there is no interrupt disable operation and even burn standard AT firmware from expressif to some esp8266 module also might encountered this issue just a short period after booup.
How to locate the cause??