silversurfer@SilverXPS:~$ python3.7 -i flatten.py >>> runit() [1, 2, 3, 4]

flatten.py

def flatlist(input):
    result = []
    for item in input:
        # It's a int? store it!
        if isinstance(item, int):
            result.append(item)
            #print(item)
        # It's a list? Run through it!
        elif isinstance(item, list):
            result += flatlist(item)
            #print(item)
    return result

def runit():
    result = flatlist([[1,2,[3]],4])
    print(result)