You say "4 smaller RAM modules". I see only two on the schematic and BOM (U6 and U7).
Now, look how they are connected: Their CS (chip select) line are shared (both connected to CS0 of CPU), and their data lane are paralleled (D0-D15 of CPU goes to the first chip, and D16-D31 goes to the second chip).
The obvious benefit of doing this is that you then have a data lane that is 32 bits wide, which provides twice the bandwidth as having a single 16-bit chip with double the capacity. The latency isn't as important as the bandwidth, usually, because there is a prefetch mechanism, some cache in the CPU, and because data is accessed in bursts.
Now, why don't they use a single 32-bit wide DDR3 chip? Because it's not easy to find. Look at digikey, they provide 42 references of 32 bit DDR3 (none of them being stocked, and most of them marked obsolete), versus 359 references of 16-bit DDR3 chips. The standard is just to use 16-bit chips.
For your application, I can't tell you what you need, however. Audio processing, even in real-time, does not usually require amazing performances, and you have something already very powerful for that. Probably, you could go with a single 16-bit chip and the bandwidth would be enough. But if you're really mixing / resampling / etc... a lot of channels, while displaying some information on a high-res screen (with the frame buffer being in the same SDRAM), you may need more. Use your logic to roughly estimate your bandwith requirements (given the audio sampling rate, number of channels, ... and, for the display: screen resolution, color depth, refresh rate, ...).