You can use tabs and spaces, you can even use both without any problem. However, your indentation has to be coherent along the whole file. So you can use as your indentation pattern "4 spaces", "one tab", "2 spaces", "one tab + 2 spaces", but you can never change your pattern in the middle of the file, once you have chosen one pattern, you have to keep this one till the end of the file or it'll throw an indentation error.

But in reality, you should indeed stick with the "4 spaces" PEP8 compliant style.

It does, and it doesn't warn you if you have both, it just breaks your program.