dv4l: 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/dv4l - from package: dv4l

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

Error in other languages:
dv4l: bevel nie gevind nie
dv4l: comandă negăsită
dv4l: comando não encontrado
dv4l: commande introuvable
dv4l: command not found
dv4l: käsku ei ole
dv4l: không tìm thấy lệnh
dv4l: komanda nerasta
dv4l: Kommando nicht gefunden.
dv4l: kommandot finns inte
dv4l: komut yok
dv4l: nie znaleziono polecenia
dv4l: níor aimsíodh an t-ordú
dv4l: no se encontró la orden
dv4l: no s'ha trobat l'ordre
dv4l: opdracht niet gevonden
dv4l: parancs nem található
dv4l: perintah tidak ditemukan
dv4l: príkaz nenájdený
dv4l: příkaz nenalezen
dv4l: команда не найдена
dv4l: командата не е открита
dv4l: 命令找不到
dv4l: コマンドが見つかりません