Python 3: starred expression to unpack a list

The *args calling convention is documented in the Expressions reference:

If the syntax *expression appears in the function call, expression must evaluate to an iterable. Elements from this iterable are treated as if they were additional positional arguments; if there are positional arguments x1, …, xN, and expression evaluates to a sequence y1, …, yM, this is equivalent to a call with M+N positional arguments x1, …, xN, y1, …, yM.

So, since you used [1, 2, 3, 4] as the expression, which is an iterable, and there were no other positional arguments, it is treated as a call with M=0 and N=4, for a total of 4 positional arguments.

You can thus also call your function as f(1, 2, *[3, 4]) or any other combination of iterable and positional arguments, provided the iterable comes after the positionals.