The 2 digit squares are: 16, 25, 36, 49, 64, 81
Thus 4 digit numbers which we can form using them are:
1616, 1625, 1636, 1649, 1664, 1681, 2516, 2525, 2536, 2549, 2564, 2581, 3616, 3625, 3636, 3649, 3664, 3681, 4916, 4925, 4936, 4949, 4964, 4981, 6416, 6425, 6436, 6449, 6464 6481, 8116, 8125, 8136, 8149, 8164 and 8181
This is when you use a caculator to square root all the numbers above and see if you get a whole number.
You will see that sqrt(1681) = 41
Hence, the answer is 1681.