Mathematics, Further Mathematics and Economics are axiomatically regarded as "hard" subjects and are favored upon by universities.
However, do check whether do that particular university accepts Further Mathematics along with Mathematics as two separate subjects. If not, you'll be one subject short of the minimum required, rendering your admission chance to total zero.
Therefore, it is advantageous to have a 4th subject as either backup, or to polish your admission resume unofficially. As in unofficially, it is widely known to the UK community that taking above 3 A2 or at most, 3 A2 1 AS, have no effect in admission. Check http://thestudentroom.co.uk, a forum for UK students in which nearly all students are replied that above the required are insane.

However, it is advisable to take 4 subjects as to impress the admission committee, or to open the options to high-tier universities such as the NUS that preferred at least 4 A2. Besides, learn from that Pakistani kid that got 30+ As in his A Levels, getting him into Harvard.

2A* 2As is nothing compared to the hordes of 4A*, as well as local candidates of Singapore-Cambridge A-Level, 4flat STPM applicants and so. Not to mention that the NUS receive outstanding applicants worldwide, one of that I notice is India.