exit script if user input not four characters

Shell Programming and Scripting
1

#!/usr/bin/bash
###script to input four characters. wxyz
echo "input first string"
read instring1
echo "input second string"
read instring2
##
echo "first string is:" $instring1
echo "second string is:" $instring2
##IF instring1 or instring2 are NOT 4 characters (xxxx) , exit 1.
##how??

If it is any easier, the four input characters "should" be numbers, like 0032 and 0198, or 2359 and 0900 , but I'd settle for trusting that the user knows to input digits and not alphas....

2 
> if [ `echo "hello" | wc -c` -gt 4 ]; then echo "too many characters"; fi
too many characters
> if [ `echo "hell" | wc -c` -gt 4 ]; then echo "too many characters"; fi
too many characters

Huh???

> if [ `echo "hell" | wc -c` -gt 5 ]; then echo "too many characters"; fi
>

That is because the echo adds a new-line after it sends the output; to start to a new line. Thus, you have to check for one more character.

3 
read -penter:\ 
case $REPLY in
  *[^0-9]* | "" ) printf "invalid number\n"; exit 1;;
           ???? ) printf "ok, let's go\n";;
              * ) printf "wrong length\n"; exit 2;;
esac

If your shell does not support read -p, use your original syntax.

P.S. Actually, this is sufficient:

read -penter:\ 
case $REPLY in
  [0-9][0-9][0-9][0-9] ) 
    printf "ok, let's go\n";;
                     * ) 
    printf "invalid number\n" 
    exit 1;;
esac