if he is literally 3betting the top 20% of hands i'd imagine you are losing a shit-ton of value by never 4-betting. obviously what he does with that range to a 4b would dictate whether you are inclined to 4b bluff him (if he folds too much) or 4b wider for value (if he calls or 5b ships too much)

ps i don't play HU tables. i could be wrong.