mod-mono-server4: 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/mod-mono-server4 - from package: mono-apache-server4

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

Error in other languages:
mod-mono-server4: bevel nie gevind nie
mod-mono-server4: comandă negăsită
mod-mono-server4: comando não encontrado
mod-mono-server4: commande introuvable
mod-mono-server4: command not found
mod-mono-server4: käsku ei ole
mod-mono-server4: không tìm thấy lệnh
mod-mono-server4: komanda nerasta
mod-mono-server4: Kommando nicht gefunden.
mod-mono-server4: kommandot finns inte
mod-mono-server4: komut yok
mod-mono-server4: nie znaleziono polecenia
mod-mono-server4: níor aimsíodh an t-ordú
mod-mono-server4: no se encontró la orden
mod-mono-server4: no s'ha trobat l'ordre
mod-mono-server4: opdracht niet gevonden
mod-mono-server4: parancs nem található
mod-mono-server4: perintah tidak ditemukan
mod-mono-server4: príkaz nenájdený
mod-mono-server4: příkaz nenalezen
mod-mono-server4: команда не найдена
mod-mono-server4: командата не е открита
mod-mono-server4: 命令找不到
mod-mono-server4: コマンドが見つかりません