tex4ht: 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/tex4ht - from package: tetex-tex4ht

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

On Debian possible default full paths are:

/usr/bin/tex4ht - from package: tex4ht

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

Error in other languages:
tex4ht: bevel nie gevind nie
tex4ht: comandă negăsită
tex4ht: comando não encontrado
tex4ht: commande introuvable
tex4ht: command not found
tex4ht: käsku ei ole
tex4ht: không tìm thấy lệnh
tex4ht: komanda nerasta
tex4ht: Kommando nicht gefunden.
tex4ht: kommandot finns inte
tex4ht: komut yok
tex4ht: nie znaleziono polecenia
tex4ht: níor aimsíodh an t-ordú
tex4ht: no se encontró la orden
tex4ht: no s'ha trobat l'ordre
tex4ht: opdracht niet gevonden
tex4ht: parancs nem található
tex4ht: perintah tidak ditemukan
tex4ht: príkaz nenájdený
tex4ht: příkaz nenalezen
tex4ht: команда не найдена
tex4ht: командата не е открита
tex4ht: 命令找不到
tex4ht: コマンドが見つかりません