xvnc4viewer: 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/xvnc4viewer - from package: xvnc4viewer

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

Error in other languages:
xvnc4viewer: bevel nie gevind nie
xvnc4viewer: comandă negăsită
xvnc4viewer: comando não encontrado
xvnc4viewer: commande introuvable
xvnc4viewer: command not found
xvnc4viewer: käsku ei ole
xvnc4viewer: không tìm thấy lệnh
xvnc4viewer: komanda nerasta
xvnc4viewer: Kommando nicht gefunden.
xvnc4viewer: kommandot finns inte
xvnc4viewer: komut yok
xvnc4viewer: nie znaleziono polecenia
xvnc4viewer: níor aimsíodh an t-ordú
xvnc4viewer: no se encontró la orden
xvnc4viewer: no s'ha trobat l'ordre
xvnc4viewer: opdracht niet gevonden
xvnc4viewer: parancs nem található
xvnc4viewer: perintah tidak ditemukan
xvnc4viewer: príkaz nenájdený
xvnc4viewer: příkaz nenalezen
xvnc4viewer: команда не найдена
xvnc4viewer: командата не е открита
xvnc4viewer: 命令找不到
xvnc4viewer: コマンドが見つかりません