What is a 4 sided polygon?

A quadrilateral.
Any ##4##-sided polygon is a quadrilateral.
take many forms but their most basic characteristic is that they are a ##4##-sided polygon.
In case you didn’t know or have forgotten a polygon is any closed figure created by a series of straight line segments.
In the case of any quadrilateral there are ##4## of these line segments.
Notice that Latin roots:
quadri- means ##4##
latus means sided