Are you planning on dropping a subject in year 13 or carrying on all 4? I did Maths, Chemistry, Biology and Geography in year 12 and have now dropped maths (I was half linear, half old style so I still have an AS) with the same predicted grades.The sciences are as I'm sure you've heard humungous subjects, and there is a lot of content! What I would say though, is that while biology probably contains the most information, it's also relatively easy stuff- the concepts arent difficult just might take a while to learn so if your time management is good then biology is probably the nicest science to do. Geography is also a large subject with all the extra case studies you have to learn, and a lot of time can be eaten up with exam practice which is really important, but as its essay questions it's annoying (40 markers this year yay!!)
It sort of depends on how much you like the subjects at the moment really, for me maths was the most time consuming because i was bad at it, and consequently it ate up a lot of my time. But geography, which i actually enjoy (shocker) is much more suited to me and even longer homeworks would take me less time- and revision was a lot easier because i was actually engaged. Maths with further maths could be a less time consuming combination for you if you're more suited to and enjoy maths. Or if you fancing doing something completely new then law would be good as it might be more interesting (I'm kind of sick of my subjects as i've been doing them for 7 years now like...)
Really though, any subject combination will be fine assuming you're at the same ability as me (judging predicited grades). It takes planning and just plain effort to get through biology, chemistry and geography and i honestly think they're like the 3 three largest subjects but there isn't that much difference between subject workload, and if you work smartly you'll be absolutely fine if you choose subjects you enjoy!!