Transform a Sum into a List

List @@ sum

{a, -b, c, d}

From the docs on Apply (@@):

f@@expr replaces the head of expr by f.

So List@@sum replaces Head[sum] (that is, Plus) with List.

You can also get the same result by changing 0th Part of sum (which is its Head) to List:

sum[[0]] = List; sum

{a, -b, c, d}

Try :

a - b + c + d /. Plus -> List

You can have a look at a - b + c + d //FullForm to see why this works.

Still another route:

Last[CoefficientArrays[#]] Variables[#] &[a - b + c + d]
   {a, -b, c, d}