Why may a man have four wives but a woman may not have more than one husband?

Polyandry (a woman having more than one husband) is forbidden in Islam; a woman who is still married is among those it is forbidden to marry.
Surah an-Nisa' 4:24 mentions "married women" (al-muhsanat) among those forbidden. Scholars explain that part of the wisdom is to preserve lineage: a child's father must be known with certainty because lineage, maintenance, guardianship and inheritance depend on it. It also keeps leadership in the household stable. This does not mean women are of lesser worth; Islam gives women the right to mahr, maintenance, their own property and the right to seek divorce if wronged. See Wisdom & criticism.