four2six: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/four2six - from package: thc-ipv6

You may install the required package with command yum install {package}

Error in other languages:
four2six: bevel nie gevind nie
four2six: comandă negăsită
four2six: comando não encontrado
four2six: commande introuvable
four2six: command not found
four2six: käsku ei ole
four2six: không tìm thấy lệnh
four2six: komanda nerasta
four2six: Kommando nicht gefunden.
four2six: kommandot finns inte
four2six: komut yok
four2six: nie znaleziono polecenia
four2six: níor aimsíodh an t-ordú
four2six: no se encontró la orden
four2six: no s'ha trobat l'ordre
four2six: opdracht niet gevonden
four2six: parancs nem található
four2six: perintah tidak ditemukan
four2six: príkaz nenájdený
four2six: příkaz nenalezen
four2six: команда не найдена
four2six: командата не е открита
four2six: 命令找不到
four2six: コマンドが見つかりません