(1) The area of the cover of the math book is about 4 square decimeters;

(2) Xiao Ming's height 135cm and room area of 30m2.

(3)4×4= 16 (square centimeter)

A: It is a square with a side length of 4 cm and an area of 16 square cm.

So the answer is: square decimeter, centimeter, square meter, square centimeter.