A level economics has very little maths in it, a few marks a paper at the most, there are a lot of graphs involved but it's all mostly theoretical. If you look up the past papers you'll see it's mostly essay Qs and stimulus responses, if you can write you'll be fine, you just need to understand the theory, none of which is impossible. Overall though I just wouldn't bother with a 4th subject, it doesn't matter to unis that you've done an extra subject, just that you've done incredibly well at at least three. It's better to get three predicted A*s than like three As and a B when you're applying, so I would focus on doing really well on 3.
On an extra note, your a levels are great if you're aiming for a humanities degree, if you do an EPQ then try and do it based on something related to what you want to study at uni, to help you get a better understanding. I don't think you'll have to write a personal statement when you apply from next year but it's essential to show extra interest in your application for oxbridge etc, away from your traditional studies e.g my friend did her epq exploring the downfall of tsarist russia and then applied for history