A polygon has 4 sides. The lengths of the sides are 4cm, 4cm, 4cm and 5 cm. This is a regular polygon.
A regular polygon is a polygon that is equiangular(all the angles are equal) and equilateral(all the sides are equal).
The sides of the given polygon are 4cm, 4cm, 4cm and 5cm.
Since all its sides are not equal in length, it is not a regular polygon.