Array Indexing and Printing in Python (pyCharm)

In main.py, I am having a hard time printing the palindrome of a string. 

When I run the commands: print("Hello"), it works

When I run: 

s="Hello"

print(s), it works

When I run: 

print(s[0:4]), it works

BUT!

print(s[4:0]) shows nothing on the command console. 

Is that the correct syntax for storing the "inverse" of a list? Why would it not print? 

I'm using pyCharm and writing the code in main.py and running it. 

Thanks!