Okay, so, here, let me hit you with this.
You're planning to set up four storage servers. Assuming your "RAID 60" translates into maybe RAIDZ2 with two groups of 10 drives (8 data, two parity) plus four spares, which may be excessively aggressive for your application. That's 16 data drives worth of space, or about 42TB of usable space per storage server, or 168TB total.
At 100Mbps, or a peak of 12.5MBytes/sec possible on your inbound connection, that's a maximum rate of about 1TB per day that can be written. Your space - ASSUMING you were able to max out your connection 24/7 starting right away - would take you about half a year to fill.
So, some free advice from someone who does this stuff for a living: rethink your business and growth plans. You are probably not well-served by going big to start. Get two 4U storage servers to start out, because you should never have just one of anything unless it's something unimportant. Doing that means that in the worst case scenario, you might be adding two more storage servers within a year, but you might not, and why waste money spinning drives that are empty? Save the capex and you will probably be able to make it stretch further in a few months, like when 4TB drives are the price 3TB's were just a little while ago.
As for the rest, pick up a pair of inexpensive managed 1Gbps switches. We're partial to the Dell 5324's for this sort of thing, they're solid and cheap on eBay. You will hopefully eventually outgrow them, at which point they'll still be useful devices for some parts of your network.
Have a competent programmer do your app development, and this specifically means NOT doing it in some crummy interpreted language like PHP. At least not for the heavy lifting.
Eventually you may want to contemplate ditching the app servers and using virtualization to run the NAS and app stuff on the same boxes; FreeNAS is lightweight on CPU and can be easy on the memory. That means a lot of spare CPU capacity can be floating around as you get up to several dozen storage servers. It helps to contemplate that up front, but you don't need to actually do anything about it. Easier to start things up the conventional way.