The side of the 4th square is

$$a_4=a_1+(4-1)d=18+(4-1)\times 18=72inches$$

The area of the 4th square is

$$Area={72}^{2}inche{s}^2$$

$$={72}^2(\frac{1}{12}feet{)}^2$$

$$=\frac{{72}^2}{{12}^2}fee{t}^2$$

$$=(\frac{72}{12}{)}^{2}fee{t}^2$$

$$=6^{2}fee{t}^2$$

$$=36fee{t}^2$$