You can't, the best you can do is 8 ohms with those speakers. Will it really sound louder at 4 ohms? No, almost no difference at all. Why?
It takes 10 times the power to double the volume (an increase of 3 db)
Changing from 8 ohms to 4 only doubles the power, and that's why there is no real hearable difference.
Now hooking up 2 cabinets instead of one cabinet? You will get a bit more efficiency, because four speakers move more air than two speakers. And it will be running at 4 ohms with two cabinets, a bit of an improvement.
And running at 2 ohms? don't do that. It will overheat and destroy the amplifier.
Actually now that I think of it, there is a way to make it better if you only can use one speaker cabinet. Take the stock speakers out and install EVM or EVX, or even JBL speakers. Then it will be louder and sound much better, and you can use two 8 ohm speakers to get 4 ohms.