The BX4210A has two 8-ohm speakers in it, connected in parallel. This creates a 4-ohm load, allowing the total 450 watts to be used for the internal speakers.
There are two 1/4 speaker cables connected to the rear panel speaker outputs of the BX4210A. One of these cables connects to both of the internal speakers, which are connected in parallel. The other 1/4 speaker cable connects the tweeter.
The total impedance of the BX4210A is already 4 ohms. It is not recommended to go below 4 ohms. To connect an external cab to the BX4210A, you will need to disconnect the internal speakers first. Alternately, you can use an additional power amp to amplify an additional speaker cab.
The manual shows an example of the BA115 and the BA210 cab connected to the BX4210A. These are both 8-ohm cabs. Two 8-ohm cabs connected in parallel gives a 4-ohm load on the amp. This setup would work with the BX4210A if the BX4210A is driving the external BA115 and BA210 cabs shown in the example, instead of the internal speakers - not in addition to the internal speakers.