81
121
It can have up to 5 digits.
81
Every square of a 4 digit number has more than four digits.
81
1
121
121 because 2x1x1=2 and 11x11=121
It can have up to 5 digits.
81
Six.
49
Every square of a 4 digit number has more than four digits.
You had me until "product." The product of 4 digits can't be prime.
81...........
121 121 = 11^2 1 x 2 x 1 = 2