If you want to challenge yourself then pick a science for your 4th subject, if not, pick something similar to Economics like Geography or Politics. Bear in mind though, A levels are much harder than GCSE and so you might find doing 4 “hard” subjects a bit overwhelming. Don’t be a masochist like me and pick all the “difficult” subjects because it looks “impressive” to universities. It doesn’t, all that matters is that you get the required grades so you can move on from this stage of your life, make it easier for yourself. Maths, Further maths, Economics, and Geography sounds like a very good combination of subjects to me.