Hello, ajfan. Winter damage could cause browning, especially if it is throughout the shrub and near the ends, which are always more sensitive to the cold. However, I would expect this to become an issue earlier in the growing season. Since you are in Z4, the timing may be ok for your zone. In case it is not, also monitor the soil moisture. Due to lack of moisture in the summer months, the leaves can turn brown from the edges inwards. Also, your PG may have attained a size where it needs more water than in prior years. Or it may need water more often.

Insects & other pests would normally damage or "skeletonize" leaves and you would have noticed this before the leaves browned out so I am not sure this is "the" problem..

Just to be on the safe side, I would also do a soil pH test using soil pH tes kits sold in plant nurseries, watch out for salt burn and do a formal soil test if you have not done one ever or not in the last 5 years or so.

Make sure you do not overwater or fertilize in response to this problem. Too much of them now can complicate analysis of the problem or cause other problems.

Luis