jmc

We'll start with the naive estimate that there are $5^4=625$ colorings, since there are 5 choices for the color of each square. Obviously, some colorings will be counted more than once. Consider a general coloring and the three other colorings obtained by rotating it. If all four squares are the same color, in 5 of the 625 colorings, we get the same thing when we rotate it, so these are not overcounted. If opposite squares match but adjacent ones do not, then we get two colorings that should be counted together, so we are double-counting these $5\cdot 4=20$ colorings (there are 5 choices for one color and 4 for the other color). In the other $5^4-5-20=600$ cases, we quadruple-count the colorings, since there are four of the original colorings that are really the same. Therefore, the total number of distinct colorings is $$5+\frac{20}{2}+\frac{600}{4}=5+10+150=\boxed{165}.$$