Should I take a fourth A-Level?

No Econ degree requires or prefers A level Econ. Its actually Maths that is the critical subject. So there is no need to take A level Econ - it wont make any difference to your application for either Econ and/or Maths.
All LSE says about A level requirements for 'Mathematics and Economics' is "A*AA, with an A* in Mathematics, Where it's offered by your school or college, AS- or A-level Further Mathematics is expected to be taken and a grade A achieved." There is no disadvantage in taking Maths and FM as 2 of your 3 A levels. There is therefore no benefit in taking an extra A level in any subject for this degree.
Taking 4 A levels plus an EPQ is not a great idea for anyone. The level of work/stress could undermine all of your grades. Remember - AAA will always look better than ABBB, and that that could cost you a Uni place. Unis are not impressed by 4 subjects - they don't think 'ooo, clever' and give preferential offers. Focus on what you are taking, focus on getting top grades - that will matter - and if the EPQ becomes too much, think about dropping it in favour of your A levels.

thanks for the advice, I'm going to try and pick up econ, and if I can't keep up with the workload i'll just drop it

I'm about a month into Year 12 so far doing Maths, Further Maths, Physics, and an Econ related EPQ. I really want to aim for a top 5 uni (Especially Cambridge or LSE) to do a degree in either maths or econ. I'm starting to really question whether it would be beneficial to me to take economics as a fourth subject. I know it will make my application look stronger, but I'm just wondering whether the extra qualification is worth the work that comes with it, bearing in mind I'll have a month's worth of work to catch up in during the half term.

I have just been to open days and am sitting maths, further maths, physics and economics plus and EPQ and they literally tell you that they ignore the fourth A-level. Its only use appears to be that if they want A*A*A you have four chances at getting those three grades (and most of them will demand A*A* in maths and further maths anyway, so its mainly about having two chances at the A.

Not all unis do this - they can ask for specific grades for particular subjects - why is your school allowing 4 + EPQ?

They mandate an EPQ and I chose the fourth A-level. Originally I wasn't sure I was going to do further maths, but I started and really enjoyed it, so I kept going. If my grades had dropped I would have dropped a subject, but they were ok, so I kept going.

Not all unis do this - they can ask for specific grades for particular subjects - why is your school allowing 4 + EPQ?

Most schools in my area allow 4 alevels + epq. Is that unusual?

Most schools in my area allow 4 alevels + epq. Is that unusual?

Private schools trying to impress potential parents?
I doubt if any Academy or Comprehensive is being this daft.

I'm about a month into Year 12 so far doing Maths, Further Maths, Physics, and an Econ related EPQ. I really want to aim for a top 5 uni (Especially Cambridge or LSE) to do a degree in either maths or econ. I'm starting to really question whether it would be beneficial to me to take economics as a fourth subject. I know it will make my application look stronger, but I'm just wondering whether the extra qualification is worth the work that comes with it, bearing in mind I'll have a month's worth of work to catch up in during the half term.