I am doing similar A-Level options as nihil_nimis (i.e. RPE with sciences, and am also doing those modules on the OCR course) and I would completely agree. I have also spoken to a number of admissions tutors, and all have said the transferable skills you learn while doing RPE, particularly the ability to approach abstract concepts and essays in a logical way, are useful in any subject. In ethics specifically, a big part of the course is about whether a universal theory of ethics for all cultures is possible, and if we can judge other cultures on our own values, something which would be very applicable to international relations.
I'm just going to add that I studied RS at GCSE, and would say that the cross-over is minimal. For the philosophy module, there is some overlap, particularly in "Arguments for the Existence of God," but there is so little detail at GCSE that you wouldn't have a problem catching up here. For the ethics, you look at issues like Euthanasia at GCSE, but you only need to provide personal views, with little justification, so a little reading around the subject (even in the news) will give you a good idea of the issues around these topics (euthanasia/ abortion etc.)
With regards to doing four A-levels, I am currently doing 5 (Maths, Further Maths, Chemistry, Physics and RPE, and I'm finding it fine. Everyone in my school has to do four A-levels anyway, so I reckon you will be alright, although, as your teachers say, you can drop out (especially if its an extra one) if the work load proves to be too much.
If you don't know if you would like it, I would recommend "The Philosophy Gym" by Stephen Law, which is a very accessible book (no prior knowledge needed) to give you a sense of what the subject is like. As for ethics, if you enjoy discussing topic issues such as abortion and euthanasia then you should be fine.
Again however, I only know about the philosophy and ethics modules, from what I am aware of the other options are more in depth studies of particular religions (and specifically their religious texts) so I think it is unlikely your school would choose this option unless it is a religious school. However, it may all be different with the new A-Levels...