How to switch a single pole light to
The system was designed with a different philosophy: rather than turn 4 lights off, dim all to 50%.  Now you want to reinvent that.  OK.  

There are two ways I can think of.  One is to rewire the ceiling, which involves a lot of fishing, luck, and/or demolition and reconstruction of the finished ceiling.  You may need a bigger switch box to accommodate 2 switches, because I have not seen duplex dimmer switches meant to fit in a 1-gang box.  

The other way is to use home-automation technology to make each lamp individually addressible.  Then use a smart switch for coarse control, or an "app" on a compatible smart device (iPod/iPhone/android) for very precise control (e.g. Lights 2, 7 and 8 on 40%, light 6 on 100%).  They don't like us to recommend particular brands or peoducts here since questions tend to be archived for a long time, but you'll find plenty on the market. 

If you're swinging toward replacing with smart fixtures, think about also making the leap to LED.  They are finally market-ready with a good selection in color temp and CRI at a sane price, considering they will outlast the house.