Mathisfun.com states that an irregular polygon is a polygon that does not have all sides equal and all angles equal. There is no specific term for a polygon that has four completely unequal sides.
The explanation provided from Mathisfun.com indicates that even though a polygon with four completely unequal sides can be called an irregular polygon, not all irregular polygons have four completely unequal sides.
According to Math.com, a polygon that has four unequal sides can also be referred to as a quadrilateral, which is simply a four-sided polygon with four angles. More common quadrilaterals include the rectangle, the trapezoid and the rhombus.