This is so mysterious that I suspect you are eager to tell us more

There is  a problem to fix quantum mechanics in to  theory of relativity concept . Because in relativity  (STR)we consider  there is  no such a thing  as simultaneous events.,But in quantum mechanics    we  consider  simultaneous things  didn't effect  to vary large distance separation.   ex-  quantum state(spine) adaptation  explain Paulier exclusive principle consider  simultaneous events exist in different frames didn't match simultaneous explained in   different frames . In other word it is possible to  exist action at distance as Newton thought about gravity.

You have some generalizations of the usual concurrence formula in vector form

(see e.g. arXiv:quant-ph/0311166)

or you could use the determinant of the reduced density matrix of a single ququad;

this is in addition SL(4)-invariant and therefore many useful features about it are known (it is e.g. related to an entanglement monotone for example (see arXiv:quant-ph/0105090)), and even generalizations to more ququads do exist

(see in arXiv:1406.0361 and eventually also arXiv:1309.6235),

at least in principle.