As you have already discovered, the only event repeat intervals available in the iOS Calendar App are:
Every 2 Weeks
If you are using iCal, you will have to adjust your desired custom repeat interval using the iCal application. Any computer capable of using the iCloud Web App calendar can also be used to adjust a custom repeat on iCloud.com.
I'm sorry, but that's really stupid. I just got an iPhone4 (so no Siri), and I find custom repeats to be one of the most useful tools iCal has to offer. I'm glad I can still set them up from iCal on my macbook pro, but it's really unnaceptable that such an essential feature is left out of the iPone version.