Yes, you can just make 4 ounces with it.
On their FAQ page: http://www.greatnorthernpopcorn.com/t-faqs.aspx
"If I have any 8 ounce kettle, do I have to make 8 ounces?
Absolutely not! One of the reasons the Great Northern Popcorn Company designed their model with an 8 ounce kettle is so 1 machine could meet the demands of a concession business and still work for the home theater. We speak from experience when we tell you that an 8 ounce kettle is a must have. Please check our ebay store for our complete line of popcorn portion packs. We stock 3 sizes for all your popcorn needs."
Microwave popcorn bags are typically around 3 ounces.
BTW, I couldn't be happier with our Great Northern machine. I also got the 8oz, and we love it. We are popping 8oz at a time right now. We do always have some left over that I toss the next day, but it's not really that expensive to begin with.
The cleaning is as hard as you make it. If you are obsessive about the kettle being perfectly clean every time, don't buy one. If you are cool with wiping it down with a paper towel, and cleaning the glass every so often, it's awesome.