man4: 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/share/man/bin/man4 - from package: filesystem

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

Error in other languages:
man4: bevel nie gevind nie
man4: comandă negăsită
man4: comando não encontrado
man4: commande introuvable
man4: command not found
man4: käsku ei ole
man4: không tìm thấy lệnh
man4: komanda nerasta
man4: Kommando nicht gefunden.
man4: kommandot finns inte
man4: komut yok
man4: nie znaleziono polecenia
man4: níor aimsíodh an t-ordú
man4: no se encontró la orden
man4: no s'ha trobat l'ordre
man4: opdracht niet gevonden
man4: parancs nem található
man4: perintah tidak ditemukan
man4: príkaz nenájdený
man4: příkaz nenalezen
man4: команда не найдена
man4: командата не е открита
man4: 命令找不到
man4: コマンドが見つかりません