startxfce4: 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/startxfce4 - from package: xfce-utils

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

On Debian possible default full paths are:

/usr/bin/startxfce4 - from package: xfce4-session

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

Error in other languages:
startxfce4: bevel nie gevind nie
startxfce4: comandă negăsită
startxfce4: comando não encontrado
startxfce4: commande introuvable
startxfce4: command not found
startxfce4: käsku ei ole
startxfce4: không tìm thấy lệnh
startxfce4: komanda nerasta
startxfce4: Kommando nicht gefunden.
startxfce4: kommandot finns inte
startxfce4: komut yok
startxfce4: nie znaleziono polecenia
startxfce4: níor aimsíodh an t-ordú
startxfce4: no se encontró la orden
startxfce4: no s'ha trobat l'ordre
startxfce4: opdracht niet gevonden
startxfce4: parancs nem található
startxfce4: perintah tidak ditemukan
startxfce4: príkaz nenájdený
startxfce4: příkaz nenalezen
startxfce4: команда не найдена
startxfce4: командата не е открита
startxfce4: 命令找不到
startxfce4: コマンドが見つかりません