Find the number of 4-lists of the form (x1, x2, x3, x4) where each xi is a nonnegative
integer and x1 + x2 + x3 + 4 * x4 = 15.

i know the equation c[(n+r-1), r) but the 4 * x4 is throwing me off a bit, wouldnt that equation over count the the max value in this case for x4 would be 3?

since it's low enough, i'd just count the range of x4 separately since x4 can only be {0, 1, 2, 3}

eg: count x1 + x2 + x3 = {15 - 4*x4} for each x4 possibility above

there's prolly some trick, but whatever