Explain Fourth group of 32 ASCII character code?

The fourth, and final, group of 32 ASCII character codes are set aside for the lower case alphabetic symbols, five additional special symbols, and another control character (delete).

Note that the lower case character signs use the ASCII codes 61h through 7Ah. If you evaluate the ASCII codes for the upper and lower case characters to binary, you will observe that the upper case symbols differ from their lower case equivalents in exactly one bit position.

The merely place these two codes differ is in bit five. Upper case characters forever contain a zero in bit five; lower case alphabetic characters always contain a one in bit five. You can use this information to quickly convert between upper and lower case and if you have an upper case character you can force it to lower case by setting bit five to one same as If you have a lower case character and you wish to force it to upper case, you can do so by setting bit five to zero and You can toggle an alphabetic character between upper and lower case by simply inverting bit five.

Certainly, bits five and six determine which of the four groups in the ASCII character set you're in:

Consider the ASCII codes of the numeric digit characters: