At renewal it's not going to be $4.98. If you're really wanting to save the $4 by starting with Yahoo instead of someplace else, you could do that and transfer later. It might be more convenient to just start off with a $9/year registrar that offers better service from the beginning.
There is a simple answer to this question: because you get (imho) not just no service, but negative service. I have tied up some of my favorite domain names with yahoo and they won't let me transfer them, and calling australia is no help at all. So, not only do they take my money, but they ruin my best domain names as well.
No telephone service to speak of, everything by strict rules and formulas, no exceptions - don't even think about losing your password. The only thing that saves you is your birthday, which everyone can find out all over the web.
I have a domain registered with yahoo, and their support is non existent. If you lose your password and forget your secret answer you are in trouble. Trying to get a coherent and applicable answer for help with specific questions is a disaster.
I'm going to try to transfer the registration of the domain somewhere else, real soon.
Having said that, I'll probably keep them as hosts for the time being.