You must take the entire exam. You do not want zeros for those two sections. If you do not take those sections you will be taking a different exam then everyone else does and schools will take this into account. A 4.5 is only the 38th percentile on the AWA so you would actually like to at least have the potential to increase that score.
Additionally, you can use the AWA and IR period to warm up, to get used to the room, to decide on using ear plugs or not,etc.
I skipped the AWA section once just because there were only 3 other people in the room (it was a holiday) and my typing sounded like fireworks going off in that empty room. I thought that these people who were trying desperately to get their scores deserved some consideration, so I went right to Quant. I felt great on the Quant section, but having not warmed up at all, I earned the lowest score that I have ever gotten on Quant - lower than any other official test and lower than any practice test.
So there are a couple of strong reasons why you need to take these sections: schools really do want to see something from you here (and you should try to improve that 4.5) and you may actually score lower on the Quant and/or Verbal sections if you have not taken the time to warm up.
Here is my advice - first, do study for the AWA and the IR. Not so much because you want to get perfect scores on those sections but more so that you can easily complete those tasks without taking too much out of yourself. The AWA can be a pretty smooth experience if you have a template and just write about the flaws in the argument that you can pick out.
For the IR, remember that it is not adaptive and so you just need to get some of the 12 questions right. Know how to approach the questions and when you come across one that is just too tough you can guess and move on. DO not try to get 12 out of 12 questions right on the IR. But getting at least half of them correct would be good.
Remember that when you are stating facts and opinions you are not using up any brainpower. It is when you are making agonizing decisions that you are taxing your brain. So don't make any agonizing decisions on AWA and IR! If things start to became painful on an IR question stop and guess at that one.
So take it pretty easy on the first two sections -- do the work but nothing painful (like a tough Quant question can be). If you skip those sections you will, in my opinion, regret it. Spend enough time practicing these tasks so that they are easy for you on test day and if you use them as a warm up they may even increase your 200-800 score.
Those are my thoughts!
