Hi, I'm Y12 as well, and literally have just been dealing with this issue.
I'm taking four subs as well (maths, bio, chem and physics) and was weighing up the idea of taking an EPQ. If you really love your subjects and want to keep them, then I would advise against an EPQ, as this will only take your time and attention away from your already heavy workload. However, if you're definitely going to drop a subject, then I would advise taking up an EPQ. At the end of the day, an EPQ can only score up to half as many points as a well done A level, so in terms of unis I'm not sure it would be good to apply with four average a levels and an average EPQ as compared to four a levels with great grades.
In regards to taking up an EPQ with an intent to drop it if you can't balance the load....I'm not sure. I would say that would be straining yourself in the easier days of sixth form, but then again it's up to you, and your work ethic and your ability to balance your health and work.
Honestly, do what would make you the most comfortable and happy. If you love your subjects, keep them. If you're desperate to do an EPQ because you really love the concept that you've come up with, then take one, only I would then advise you to drop a subject. But do what makes you happy and take the option that won't affect your health.
I've just read that over and I seem to be really repetitive....sorry about that ahaha.