For a 4 x n grid, we think the rule stands that the number of sides is 4n where n is greater than or equal to 4.
So for a 4 X 4 grid the number of sides is 16.
For a 4 x 5 grid the number of sides is 20.
For a 4 x 6 grid the number of sides is 24.
BUT:
For a 4 x 2 grid the number of sides is 4 x 2 - 4 = 4
For a 4 x3 grid the number of sides is 4 x 3 - 2 = 10 (this fits the n x 3 pattern of 3n -2)
For a 5 x n grid, we think the rule stands that the number of sides is 5n where n is greater than or equal to 4.
BUT:
For a 5 x 2 grid the number of sides is 5 x 2 - 6 = 4 (the amount we subtract for an n x 2 grid goes up by 2 each time n goes up by 1).
For a 5 x 3 the number of sides is 5 x 3 - 2 = 13 (this fits the n x 3 pattern of 3n-2).
SO:
When n is less than 4 it always follows the patterns we have described above for n equals 2 or 3.