well I am sure that you probably know this but when water heats up it wants to expand, and I use a standard cap to keep it under preasure, like 12-16lbs and my engine runs great, not to meantion that if you only have four lbs the second that hot water comes out it want to escape due to the increased preasure and if the cap isnt rated high enough it will keep doing that, not to mention that at four lbs the boiling temp is much lower, if you increase the rating on your cap you'll be a lot happier, it wont leak and your car will run cooler and better because it wont let the water expand which keeps it cooler and you boiling temp higher, I tried this a ways back with like a 6lbs cap and it did the same thing and I just wanted to shoot myself, so finally I just fixed my rad leak and put a standard 16lb cap on it and have loved it ever since, give it a shot and see what you get.
