The BX4500H has a 4-ohm minimum, allowing the total 450 watts to be used for the speakers.
It is not recommended to attempt to drive a load lower than this four ohm minimum with the BX4500H. Doing so could cause too much current draw and overheat the unit. Alternately, you can use an additional external power amp to amplify an additional speaker cab.
The manual shows an example of the BA115 and the BA210 cab connected to the BX4500H. These are both 8-ohm cabs. Two 8-ohm cabs connected in parallel gives a 4-ohm load on the amp.