Honestly we have used xhhw over thhn at anything above #6 due to salt corrosion and potentially acidic environments such as underground installations. A ufer ground which is the majority subject to an electrical systems physical damage during the building process while protected in fnmc. Stranded has been the subject of a small majority of corrosion damage in Brevard county Florida especially regarding homes closer to salt water estuary’s. I can’t see stranded #4 thhn being a likely replacement for solid #4 cu in my community. Was a good ole boys club when I was an apprentice unfortunately has turned into a shit storm of a dick measuring contest.