Giles wrote:
Six, four letter words. All six use the same four letters. Four
different letters.....no repeats. All six words are in very common
English usage......at least one is seen by the vast majority of
American English speakers (and, presumably, a very high percentage of
the rest as well) virtually every day, spelled out in large sans serif
block letters. None of the six has any commonly accepted variant
spellings.

What are the six words?

g.
who, after lo these many years of desultory attention, knows of only
one set of four letters that will satisfy all of the above
requirements.

STOP
POTS
TOPS
POST
SPOT
OPTS