designer-qt4: 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/bin/designer-qt4 - from package: qt-devel
/usr/lib64/qt4/bin/designer-qt4 - from package: qt-devel
/usr/lib/qt4/bin/designer-qt4 - from package: qt-devel

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

On Debian possible default full paths are:

/usr/bin/designer-qt4 - from package: qt4-designer

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

Error in other languages:
designer-qt4: bevel nie gevind nie
designer-qt4: comandă negăsită
designer-qt4: comando não encontrado
designer-qt4: commande introuvable
designer-qt4: command not found
designer-qt4: käsku ei ole
designer-qt4: không tìm thấy lệnh
designer-qt4: komanda nerasta
designer-qt4: Kommando nicht gefunden.
designer-qt4: kommandot finns inte
designer-qt4: komut yok
designer-qt4: nie znaleziono polecenia
designer-qt4: níor aimsíodh an t-ordú
designer-qt4: no se encontró la orden
designer-qt4: no s'ha trobat l'ordre
designer-qt4: opdracht niet gevonden
designer-qt4: parancs nem található
designer-qt4: perintah tidak ditemukan
designer-qt4: príkaz nenájdený
designer-qt4: příkaz nenalezen
designer-qt4: команда не найдена
designer-qt4: командата не е открита
designer-qt4: 命令找不到
designer-qt4: コマンドが見つかりません