I’ve seen both and never been sure if one was more old fashioned or less “courant” than the other.
In text documents, un de ces jours is much more common, even when including un de ces quatre in the comparison :
Un de ces quatre, being more colloquial, is certainly more frequent in spoken French than in books, but I believe un de ces jours still leads in this context although I have no data to back it up.
"A un de ces quatre (matins)" is only used with good friends. While "A un de ces jours" can be said to anyone.
Tu le reverras "un de ces quatre (matins)" est plus familier et "un de ces quatre" encore plus. "A un de ces jours!", c'est pour ne pas dire "Au revoir!"