Riffle multiple lists together

As mentioned in the comments, Tuples[] does it:

Tuples[{{a, b}, {1, 2, 3, 4}}]
   {{a, 1}, {a, 2}, {a, 3}, {a, 4}, {b, 1}, {b, 2}, {b, 3}, {b, 4}}

Before Tuples[] became a built-in function, one used to use Outer[] for the task, in conjunction with Flatten[]:

Flatten[Outer[List, {a, b}, {1, 2, 3, 4}], 1]

There is also: Distribute

Distribute[{{a, b}, {1, 2, 3, 4}}, List]

{{a, 1}, {a, 2}, {a, 3}, {a, 4}, {b, 1}, {b, 2}, {b, 3}, {b, 4}}