Xvnc4: 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 Debian possible default full paths are:

/usr/bin/Xvnc4 - from package: vnc4server

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

Error in other languages:
Xvnc4: bevel nie gevind nie
Xvnc4: comandă negăsită
Xvnc4: comando não encontrado
Xvnc4: commande introuvable
Xvnc4: command not found
Xvnc4: käsku ei ole
Xvnc4: không tìm thấy lệnh
Xvnc4: komanda nerasta
Xvnc4: Kommando nicht gefunden.
Xvnc4: kommandot finns inte
Xvnc4: komut yok
Xvnc4: nie znaleziono polecenia
Xvnc4: níor aimsíodh an t-ordú
Xvnc4: no se encontró la orden
Xvnc4: no s'ha trobat l'ordre
Xvnc4: opdracht niet gevonden
Xvnc4: parancs nem található
Xvnc4: perintah tidak ditemukan
Xvnc4: príkaz nenájdený
Xvnc4: příkaz nenalezen
Xvnc4: команда не найдена
Xvnc4: командата не е открита
Xvnc4: 命令找不到
Xvnc4: コマンドが見つかりません