xfce4-about: 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/xfce4-about - from package: xfce-utils

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

On Debian possible default full paths are:

/usr/bin/xfce4-about - from package: libxfce4ui-utils

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

Error in other languages:
xfce4-about: bevel nie gevind nie
xfce4-about: comandă negăsită
xfce4-about: comando não encontrado
xfce4-about: commande introuvable
xfce4-about: command not found
xfce4-about: käsku ei ole
xfce4-about: không tìm thấy lệnh
xfce4-about: komanda nerasta
xfce4-about: Kommando nicht gefunden.
xfce4-about: kommandot finns inte
xfce4-about: komut yok
xfce4-about: nie znaleziono polecenia
xfce4-about: níor aimsíodh an t-ordú
xfce4-about: no se encontró la orden
xfce4-about: no s'ha trobat l'ordre
xfce4-about: opdracht niet gevonden
xfce4-about: parancs nem található
xfce4-about: perintah tidak ditemukan
xfce4-about: príkaz nenájdený
xfce4-about: příkaz nenalezen
xfce4-about: команда не найдена
xfce4-about: командата не е открита
xfce4-about: 命令找不到
xfce4-about: コマンドが見つかりません