convert4r4: 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 CentOS possible default full paths are:

/usr/sbin/convert4r4 - from package: exim

You may install the required package with command yum install {package}

Error in other languages:
convert4r4: bevel nie gevind nie
convert4r4: comandă negăsită
convert4r4: comando não encontrado
convert4r4: commande introuvable
convert4r4: command not found
convert4r4: käsku ei ole
convert4r4: không tìm thấy lệnh
convert4r4: komanda nerasta
convert4r4: Kommando nicht gefunden.
convert4r4: kommandot finns inte
convert4r4: komut yok
convert4r4: nie znaleziono polecenia
convert4r4: níor aimsíodh an t-ordú
convert4r4: no se encontró la orden
convert4r4: no s'ha trobat l'ordre
convert4r4: opdracht niet gevonden
convert4r4: parancs nem található
convert4r4: perintah tidak ditemukan
convert4r4: príkaz nenájdený
convert4r4: příkaz nenalezen
convert4r4: команда не найдена
convert4r4: командата не е открита
convert4r4: 命令找不到
convert4r4: コマンドが見つかりません