qtdemo-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/qtdemo-qt4 - from package: qt-demos
/usr/lib64/qt4/bin/qtdemo-qt4 - from package: qt-demos

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

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