How do I run my B&k 200.2 s2 at 4 ohms

You don't switch it. It just is rated for 4 ohm or 8 ohm loads with output power. If you want to run at 4 ohms, hook up 4 ohm speakers...
The other option is if you want to push it, you can run a pair of 8ohm speakers off each channel in parallel which will look like 4 ohms to the amp (not the greatest suggestion). DJ gear is a different animal.
Why would you want to switch it to 4ohms anyway?

Just plug it in, and connect the speakers. It's not like a tube amp or pro gear - it has no output transformers with 4 ohm taps. It just deals with the load presented to it - and is designed to do so.