Question
My 4ft high bush, planted one year ago is turning brown in the middle and is slightly yellowing on one side (west) it gets sun from about 10am to dusk.  I have already lost one and cant find any explanation from nurseries in my area.  

Answer
Howdy Kathy I'm Coach Dave and I work for the Home Depot in California,
Shrubs are my area of expertise so let's see what we can do.
 The Arborvitae is usually a fairly hardy shrub. In regards to the yellowing here are the usual causes:
   Over watering: Once or twice a week for 10 to 15 minutes is enough.
   Poor Drainage: Make sure the water has somewhere to go. The ground should be soft not hard and sloped away from the shrub.
   Nutrient deficiency: a basic evergreen fertilizer will go a long way.

 As for the browning this is a more serious problem. Spider mites are the main culprits associated with this.  Simply cut off the brown parts then spray the entire plant with a miticide. These are usually in the form of an oil or pyrethrum based spray. Spray the plant every five days two to three times. Make sure the surrounding ground is treated as well.
I believe this will solve your Arborvitae's problems, and if any problems arise again let me know. (pictures are helpful).
Yours Respectfully,
Coach Dave

揑m a Home Depot Associate, trained and authorized to help people on the Internet."