Discuss and learn French: French vocabulary, French grammar, French culture etc.
Intuitively I'd say that "un de ces quatre matins" (which is an ellipsis and comes from 'un de ces quelques matins') means that something is down to happen quite soon whereas "un de ces jours " refers to something that might happen anytime (sooner or later). But you could use one or the other without affecting the overall meaning of the sentence.
Both are commonly used in France.
I am more familiar with "un de ces quatres" which I think means the same thing as "un de ces quatres matins".
I have read that "un de ces quatres" is an ellipsis for "un de ces quatre matins" but I am not sure that 'un de ces quelques matins' has the same meaning-or would be much used.
I can't tell you if the expression is used universally in the French speaking areas.
"un de ces quelques matins" is the origin of the expression then "quelques" turned into "quatre" so yes you're right "un de ces quelques matins" is no more used
There is also "à un de ces quatre"which means see you soon