If you want to do a maths heavy degree further maths is a very helpful 4th/3rd choice .
I do further maths, maths, economics and history and I love my subjects. The economics teaching is naff but further maths re inspired a liking of maths in me after gcse being just so boring. What I would say is if ur not on a high 8/9 at gcse further maths will be very tough, (still tough even with a 9 with no revision) . It definitely makes the normal a level look ridiculously easy though in comparison. I’ve done all of as level apart from binomial distribution and mechanics and will finish the as by Christmas so you definitely have a lot of work when you do 4 including maths but it could easily be well worth it for you(just don’t be afraid to drop a subject ).
History and biology are both very content heavy but the difference is the essay skills required in history are like 40% of your grade . It’s tough(my most difficult subject) but I love it and the content is much more interesting than gcse (the Tudors are suddenly very interesting and I’m going to germany!!!!). Biology from what I know from friends is just extremely content heavy and in October they showed me about five stacks of flash cards which I think was already more than the entire gcse if not close . But it seems interesting even as someone who massively prefers humanities and maths and disliked gcse biology.
Now whether you do 4 a levels is up to you but like I said further maths would be very helpful for a maths degree so if you can work hard consistently , then trying it out at least is well worth it.