When an unsigned int and an int are added together, the int is first converted to unsigned int before the addition takes place (and the result is also an unsigned int).
-1, while being the first negative number, is actually equivalent to the largest unsigned number - that is, (unsigned int) -1 === UINT_MAX.
-2 in unsigned form is UINT_MAX - 1, and so on, so -40 === UINT_MAX - 39 === 4294967256 (when using 32bit ints).
Of course, adding 4 then gives your answer:
4294967256 + 4 = 4294967260.
This is a great quiz where you can learn some of the rules of integers in C (and similarly C++): http://blog.regehr.org/archives/721
