one line of the code about password question can't understand

See more:

C

hi guys!

I really don't konw this!

a c program!

Password question: turn the letter A into letter E, a to e, namely into the fourth letter of the following, turn the W into A, X into B, Y into C, Z into D. For example: the transfer "China" to "Glmre".

Procedure is as follows:

#include<stdio.h>
void main()
{
   char c;
   while((c=getchar())!='\n')
           {
              if((c>='a'&&c<='z')||(c>='A'&&c<='Z'))
                  {
                      c=c+4;
                       if(c>'Z'&&c<='Z'+4||c>'z')
                            c=c-26;
                   }
               printf("%c",c);
            }
     printf("\n");
}

my question is:In the first ten lines of code, why the processing of lowercase letters without written c > 'z' && c < = 'z' + 4, but just write c > 'z'?

hope you can help me!thank you