Specks of kryptonite, gorgonite, kraterite, or reddite, or sometimes all four, can cause Superman to become sick.
Should there be a comma after "four"?
I know that in most sentences we do not include a comma after the word "or" such as the following example. John, Jane, or Sally will come to the party.
But I'm wondering if in the first example, there is some special rule about using the phrase "sometimes all four" which makes it a non-restrictive clause.