xflock4: 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/xflock4 - from package: xfce-utils

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

On Debian possible default full paths are:

/usr/bin/xflock4 - from package: xfce4-session

You may install the required package with command apt-get install {package}

Error in other languages:
xflock4: bevel nie gevind nie
xflock4: comandă negăsită
xflock4: comando não encontrado
xflock4: commande introuvable
xflock4: command not found
xflock4: käsku ei ole
xflock4: không tìm thấy lệnh
xflock4: komanda nerasta
xflock4: Kommando nicht gefunden.
xflock4: kommandot finns inte
xflock4: komut yok
xflock4: nie znaleziono polecenia
xflock4: níor aimsíodh an t-ordú
xflock4: no se encontró la orden
xflock4: no s'ha trobat l'ordre
xflock4: opdracht niet gevonden
xflock4: parancs nem található
xflock4: perintah tidak ditemukan
xflock4: príkaz nenájdený
xflock4: příkaz nenalezen
xflock4: команда не найдена
xflock4: командата не е открита
xflock4: 命令找不到
xflock4: コマンドが見つかりません