vnc4server: 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/vnc4server - from package: vnc4server

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

Error in other languages:
vnc4server: bevel nie gevind nie
vnc4server: comandă negăsită
vnc4server: comando não encontrado
vnc4server: commande introuvable
vnc4server: command not found
vnc4server: käsku ei ole
vnc4server: không tìm thấy lệnh
vnc4server: komanda nerasta
vnc4server: Kommando nicht gefunden.
vnc4server: kommandot finns inte
vnc4server: komut yok
vnc4server: nie znaleziono polecenia
vnc4server: níor aimsíodh an t-ordú
vnc4server: no se encontró la orden
vnc4server: no s'ha trobat l'ordre
vnc4server: opdracht niet gevonden
vnc4server: parancs nem található
vnc4server: perintah tidak ditemukan
vnc4server: príkaz nenájdený
vnc4server: příkaz nenalezen
vnc4server: команда не найдена
vnc4server: командата не е открита
vnc4server: 命令找不到
vnc4server: コマンドが見つかりません