I'm gonna be doing econ at uni next year and took econ, maths, f.maths and history.
As far as I know, the only university that has stated a strong lack of preference for further maths is LSE, to the point where if you want to apply for straight economics at LSE, you need 3 a-levels, excluding further maths even though further maths is appreciated by them. If you take maths, f.maths and econ then you can still apply for maths and economics at LSE though.
Taking further maths and doing well is a good indicator for universities given that economics at university has a LOT of maths in it, and doing further maths can make your first year easier compared to those who haven't taken it at a-level depending on the course. However, further maths is significantly harder than normal maths so be prepared to work for this one far more than your normal maths a-level.
However, Geography is also very related to economics so that is also helpful.
Imo, if you can, take all 4. Doing so gives you the variety and the benefit of the maths proficiency unis like.
If you are worried that doing 4 could lead to you getting lower scores, then I'd say maths, econ and geography because I think the variety is quite important - are you okay doing maths for 2/3rds of your a-level life? Also, given what I've said about LSE, if you want straight econ at LSE you can't just do maths, f.maths and econ.
Perhaps try talking your form tutor or a teacher you feel comfortable with and ask them how they think you will cope with 4 a-levels. If they are experienced they will have seen other students go through your process and seen how some cope and some struggle a bit.