I planted 4 emerald arborvitaes in May. I live in central CA and it is regularly hot in summer months. I have them on a drip timer for 20 minutes twice daily. The ground around the roots constantly looks moist. Are they not getting enough water? Could I be over watering? Could it be a soil deficiency? Not sure what the issue could be. I have some pictures I have attached as well. Any advice would be greatly appreciated. Thank you.

Welcome to NGA!
Newly planted shrubs need regular good soaking, especially if they were planted in the summer. They would have been better off if you had waited for cooler conditions as hot summers and drought are not an ideal time to plant these. A drip may not be adequate enough to reach the roots while they are trying to get established. They must receive plenty of moisture during the growing season and be watered more frequently and deeply during drought and hot summer conditions. The soil on top may be moist but what does it look like at the root ball? Did you add anything to the hole when planting to loosen up the soil so that the water can reach the roots?
Once they are that brown, sadly they won't recover their green.

Soil might be a problem, too. I have tried these twice and they don't like our heavy clay.

Hi!

Soil plays a major role in plant success or failure, and where I live arborvitae are finicky but to ensure a good start, plant food with acid is recommended, you may want to give it a try. Good luck.