First, I would highly recommend not self studying 4 AP tests. If you have enough time on your hands to be self-studying 4 APs (two of which you self described as being not easy), you can be using that time to do other things. If you are interested in history and politics, you will be better off spending the time doing extracurriculars related to history and politics, perhaps working at your local historical society or volunteering with a political campaign.
That being said, I don’t think it hurts to be self-studying an AP. It is a great way to learn something new and to have an end goal in mind. I self-studied this past year (read: taking a practice exam the night before the actual test and looking over the basic concepts two days before the test) for a topic I am interested in because I wanted to see how much I knew. Other than that, I’m not going to get much out of having self-studied. It did make me elgible for AP scholar with distinction, but, to be honest, I don’t know if that even matters much to colleges. My guess is that even that doesn’t really matter.
Self-studying might play a different role, however, depending on which APs your school offers. If you are planning on self studying AP euro and your school has AP euro, you need to be taking that class and doing well in that class. Colleges are looking for students who do well in the classroom, not always the ones that can do well on a standardized test. The other side of that coin also is that if you school does not offer AP euro, but a regular or honors euro history class and you do well in that class, taking the AP test would be a great way of showing that not only did you could do well in the classroom, but you can master it to a college level.
Also of consideration is what score you might get on the test. If you are a student looking at a top college, getting a 3 on a self-studied exam is not going to help at all. You want to be getting a 5 (even a 4 might be pushing it…). Overloading on 4 self-studied AP exams definitely puts you at risk of not top scores. But if you are looking at insitutions that will accept credit for getting a 3 or a 4, then taking those 4 self-studied APs likely will benefit you (and could save you a lot of classes!)
And final bit for you to consider is your past experience with standardized testing and APs. Some people simply aren’t the best test takers-- if that’s you, don’t push the self-studied APs. If you have taken APs before and have done well, you are in a better position for self-studying for those tests. That is especially true if you have taken APs before in the same realm as the ones you are self-studying-- if you’ve only ever taken science APs, taking a self-studied history AP is going to feel a lot different. And to that all, the flip side is true. If you haven’t taken APs before or haven’t done well, I would be extremely hesitant about self-studying one AP, let alone four.
There are my thoughts on it. Without knowing more about you or your situation, I would say just self-study one.