It's zero indexed.
Your program will not do what you want it to, but you have the right idea.
So when you ask for spot 4, you think you'll get an "s", but rather you'll get a space because it counts starting at 0 and index 4 is a " ".
This may be what you meant:
String string1 = "This is string1";
String string2 = "This is string2";
String prompt = "The 4th Char in string1 is ";
charAt4 = string1.charAt(3); //Prints 4th spot, the s
//charAt4 = (String)string1.charAt(3);