Even thouh Im new to Aikido I think I can take an honest standpoint on the issue. In all honesty I like the idea of earning rank. It shows achievements you have made, and make syou feel proud when you work twords a goal.
I think the rule of not asking has to do with humbleness. I think the best thing for people to do in a situation where they feel they are ready to test, but the sensei hasn't told them to, is to try casualy mentioning it to your sensei without coming right out and saying" Im ready for another rank. Say something like " Sensei, I was wondering what you look for when you grade people for (insert next rank up here) test?"
I think he will get the hint, even if it is indirectly and let you know when you are ready.
My 2 cents
I hope you do well on your 5th kyu test and I wish you the best of luck!