Talk to your local rep, I bet they'll let you get your CAT-4 license. That's what I'd do. Not quite as sketchy as the 5s, and you won't be really sandbagging.
EDIT: Also, an additional word - hubris.
Think about if you enter the CAT-5 race and get dropped or perform badly...what'd that do to your psyche? If you get dropped in a 4 or 3 race you will be able to better justify your performance