Flatten list sublists

[chesschaser]

Hi guys! I have this list:

 mylist = ["Hello", ["Hello2", "Hello3"], "Hello4"]

However, I want the list to be this instead (don't say just remove the sublist. This is not the real list. Just a test list. The real list comes from a function return so that wouldn't work):

 mylist = ["Hello", "Hello2", "Hello3", "Hello4"]

I have tried this:

sum(map(str.split, sum([[k] if isinstance(k, str) else k for k in single_list1], [])), [])

However this will not work as the real list has spaces and this splits them into different list items too which I do not want.
I have tried a number of different functions in place of str.split however none worked.

Please help