The zone map is based on temperatures over a ten or more year period. Local conditions can create warmer or cooler areas inside a zone, called microclimates. Zone designations are sometimes done by the original hybridizer or grower and are based on their observations. They may not be accurate for all areas.  The arboretum is in zone 4b. See this website. 
https://northerngardener.org/find-your-hardiness-zone/
You may want to ask the arboretum staff about witer protection for specific plants.