What variable name should I use after Z if I am specifying coordinates in four dimensions?

Justin Rising is correct for the convention in mathematics.

As to what the 27th letter of the Roman Alphabet is: Thorn (letter) has been argued for.

In Greece, where are popular countries for Greek professionals to migrate to escape the Greek economic problems?

There’s a trickle of Greeks coming into Australia, mostly based on preexisting family connections  (or returnees whose children have ended up going back). The cafés of Oakleigh, Victoria  (the current Greektown of Melbourne) are awash with waiters speaking contemporary Greek slang, rather than codeswitching from 1950s village patois.

But Australia is, as my grandmother once said, πολλά αλάργο,* and most Greeks have indeed fled to Western Europe.

* Cretan dialect for “very far”. αλάργο is Italian a largo.

Do I need a good understanding of mathematics in order to excel at linguistics?

IMHO: for most disciplines no.

Steve Rapaport has spoken on Applied Linguistics; but Applied Linguistics is a very different discipline to Theoretical/General. Phonetics is an experimental science, so you’ll need statistics there.

Reconstructing in historical linguistics requires a degree of rigour and thinking in terms of rules which is a bit like maths, but only a bit. Ditto phonology and syntax, and I guess morphology. The other branches not so much.

Formal semantics has much in common with mathematical logic. But you’ll get more out  of philosophy than mathematics if you go that way.