Because you added the integer value 4 to the char value 'a'. When you do that, your char value is converted to the equivalent integer value. The way that's done is to use the character's Unicode code point, which is 97. Adding 4 gives you 101. And then you cast it back to a char value, that produces the character whose Unicode code point is 101, namely 'e'.
Here's a link to a page which describes some Unicode characters, including the Latin character set:
List of Unicode characters. Note that the code points are expressed in hexadecimal.
I'm not sure why you're doing that, though, since it doesn't have anything to do with solving quadratic equations.