xfhelp4: 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/xfhelp4 - from package: xfce4-doc

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

On Debian possible default full paths are:

/usr/bin/xfhelp4 - from package: libxfce4ui-utils

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

Error in other languages:
xfhelp4: bevel nie gevind nie
xfhelp4: comandă negăsită
xfhelp4: comando não encontrado
xfhelp4: commande introuvable
xfhelp4: command not found
xfhelp4: käsku ei ole
xfhelp4: không tìm thấy lệnh
xfhelp4: komanda nerasta
xfhelp4: Kommando nicht gefunden.
xfhelp4: kommandot finns inte
xfhelp4: komut yok
xfhelp4: nie znaleziono polecenia
xfhelp4: níor aimsíodh an t-ordú
xfhelp4: no se encontró la orden
xfhelp4: no s'ha trobat l'ordre
xfhelp4: opdracht niet gevonden
xfhelp4: parancs nem található
xfhelp4: perintah tidak ditemukan
xfhelp4: príkaz nenájdený
xfhelp4: příkaz nenalezen
xfhelp4: команда не найдена
xfhelp4: командата не е открита
xfhelp4: 命令找不到
xfhelp4: コマンドが見つかりません