for {set i 0} {$i <16} {incr i} {
    set hex_str [format "%04b" $i]
    puts -nonewline "hex_str = $hex_str"

    puts -nonewline "  bit\[3:0\] ="
    for {set j 3} {$j >= 0} {incr j -1} {
        set idx [expr 3-$j]
        set bit_str [string range $hex_str $idx $idx]
        puts -nonewline "  $bit_str"
    }
    puts ""
}