Business Studies isn't an advantage to you, but take it if you enjoy it. Possibly just to AS. They tend not to care about your 4th A Level, it can be unrelated. But BS won't help you as another subject might.
However, they do not respect Computing (see it the same way as ICT). The only computer-themed A Level they like is Computer Science but they don't expect it as not many schools offer it, you're not at a disadvantage if you don't take it!
I'd recommend Physics, Maths, Further Maths.
Then, you should chose a 4th you enjoy or that is loosely related. The 4th doesn't matter much, and you can drop for AS. Maybe another science, like Chemistry, or something interesting like Economics?
As for how Further Maths works...
Maths - 4 core modules (C1, C2, C3, C4) + 2 applied (Mechanics 1/2/3, Statistics 1/2/3, Decision 1/2/3)
Further Maths - 2 core modules (FP1, FP2) + 4 applied (Mechanics 1/2/3, Statistics 1/2/3, Decision 1/2/3, Further Pure 3/4/5)
Obviously if you do a module in Maths, you can't do it in Further Maths.
Best of luck!