It depends a bit on what you mean by "IPTV".
If you have a provider, you can use a plugin like Xtreamity.If you have individual stream URL's, you can create bouquet entries manually (either by editing the bouquet file from the commandline when Enigma is stopped, or using a bouquet editor that supports streams).
Either way, the ET4000 is an antique with a slow processor, and as streaming may require lots of CPU (depending on the media type), it may not be the experience you expect.
Also, it might struggle running a recent version of OpenPLi, due to the lack of available flash memory. To update it, you need to flash again (use the menu option, and opt for a backup on USB), there is no room to do software updates.