Hi, I planted 20 emerald green arborvitae trees 4 years ago, they all did well until last winter when I found some of the trees partially turned brown. It looks like getting a little bit better during the summer time. But I found out all the trees have brown needle outside. What is the problem?

All evergreens will develop brown needles in the interior of the plant as newer needles form each year. This is expected. If your evergreens and browning from the outside in, including the current years growth, this is not normal. The fact that they all are experiencing this condition would point towards environmental pressure that has caused this current state of decline. Your past winter could have created trunk cracking near the soil line, they could have gone through the winter water stressed, etc. I suggest you contact your local extension office for a more local focused approach to find out what has happened.