K,
is it the most negative number you are seeking? And it has to be
negative? If so, this formula will produce the correct header:
=INDEX(A1:D1,MATCH(MIN(IF(A2:D2<0, A2:D2, 1)), A2:D2, 0))
It must be array entered, i.e. Shift+Ctrl+Enter

HTH
Kostis Vezerides