I just got this green velvet boxwood in a pot. The pot height is 13 inch. The sides of the square are 9 inch at the top and 7 inch at the bottom. Please see photo attached. My questions are:

1. can I keep it in its current pot? Ideally I like to maintain its current size but I'm worried that it's not going to survive in the winter. I live in zone 5 and the label on it this boxwood says it's hardy for zone 4. I live in an apt so will have to keep it in the balcony over winter.

2. If I repot the boxwood, can I use promix premium potting soil? I have a big bag at home. Do I need to mix it with perlite or something for better drainage?

thanks for your advice!