Liquid Smoke; Hi ! Welcome to the forum..
I'll be the one to be the harbinger if ill news.
Under the given prerequisites there is no way to add another partition, the limit is four partitions per disk. Google about to find the technical details. Something will have to give in order to get a partition to install ubuntu onto.
There are ways to arrange this. Has been done many times (delete the recovery partition after copying it to cd is the most prevalent). Search this forum for ideas/instructions/tutorials.
hope to see ya on ubuntu !