The numbers $a,$ $b,$ $c,$ $d$ are equal to 1, 2, 3, 4, in some order.  Find the largest possible value of
\[ab + bc + cd + da.\]
We can factor $ab + bc + cd + da$ as $(a + c)(b + d).$  Then by AM-GM,
\[(a + c)(b + d) \le \frac{[(a + c) + (b + d)]^2}{4} = \frac{10^2}{4} = 25.\]Equality occurs when $a = 1,$ $b = 2,$ $c = 4,$ and $d = 3,$ so the largest possible value is $\boxed{25}.$