Can I hook up external speakers to my flat screen computor monitor which hase internal speakers?

David is on the right track, as the disabling of the internal speakers prevents overload. If you connect two eight ohm speakers per stereo side, you will pull a four ohm load and damage the system, the speaker crossover of one or both speakers (if they exist), or just blow a fuse, if lucky. The correct way is to route speaker output to an external amp which you set up properly, using the correct speakers for your load.

Example two: four sixteen ohm speakers (two per stereo side) will equal eight ohms, which is probably the proper speaker load your system puts out. If you were to use four of these sixteen ohm speakers, yes, you can run four speakers, either opening things up and soldering (bad choice) or using the proper stereo plugs to "y" the speaker setup you want. I'd go with an amp, like a one used for multiple entertainment sources, t.v.,stereo, computer, etc. They generally put out a four ohm load which sounds like what you need, if you already have eight ohm speakers. Or, just follow David's advice and plug in with no worries. They knew you were coming and safely provided the disable feature to protect everything when you got that "bright" idea. You could buy a cheap pair of powered speakers and turn the volume up better, but it won't be a big subwoofer system, of course.

Usually plugging in external speakers to the headphone jack disables the internal speakers, there is nothing you can do about this