pdp4: 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/pdp4 - from package: simh

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

Error in other languages:
pdp4: bevel nie gevind nie
pdp4: comandă negăsită
pdp4: comando não encontrado
pdp4: commande introuvable
pdp4: command not found
pdp4: käsku ei ole
pdp4: không tìm thấy lệnh
pdp4: komanda nerasta
pdp4: Kommando nicht gefunden.
pdp4: kommandot finns inte
pdp4: komut yok
pdp4: nie znaleziono polecenia
pdp4: níor aimsíodh an t-ordú
pdp4: no se encontró la orden
pdp4: no s'ha trobat l'ordre
pdp4: opdracht niet gevonden
pdp4: parancs nem található
pdp4: perintah tidak ditemukan
pdp4: príkaz nenájdený
pdp4: příkaz nenalezen
pdp4: команда не найдена
pdp4: командата не е открита
pdp4: 命令找不到
pdp4: コマンドが見つかりません