The first step is to calculate the length of the diagonal of the square opening.
The length of a diagonal of square = 
 * side
 * side
Given side = 3 feet
Length of the square hole = 
 *3 = 4.24 feet
 *3 = 4.24 feet
since the diagonal is 4.24 feet and the shorter side of the plywood piece is only 4 feet, we can slide it through the square hole diagonally 
Answer is YES since the diagonal 4.24 feet is greater than the shorter side of the plywood piece *4 feet