I use an attenuator I got here:
http://www.electronicplus.com/prodSe...&Submit=Search
but I don't know if you'll be able to get it by Saturday.
Can you use the headphone out of your mixer, or some other output that has a level control?
As you've noted, the 1/4" inputs are "instrument" level rather than true line level.
If you can solder you can build an attenuator out of a few resistors or an adjustable one using a couple of pots. I've also used a cheap(ly made) $15 headphone volume control from Radioshack, but it will need a bunch of adapters to get from the mixer out to the H4n 1/4" in.
The lack of real input attenuation is my nomination for the biggest flaw in all the Zoom recorders I've tried, and I've used a bunch of them.
Fran