Here's the basic setup of my network

In this diagram:

1: The internet
c: cable
2: Wireless router
w: wireless connection
3: A win7 box with internet connection sharing enabled
4: A wireless router, but I'm only using its LAN capabilities to connect box 5 to the internet.
5: A win7 box, the computer I'm using to make this post. So its internet works just fine.

Now if I'm on box 5, and I ping 192.168.1.1, I hit 4. If I'm on box 3 and I ping 192.168.1.1, I hit 2. Now obviously box 3 does not think 4's IP address is 192.168.1.1, or I wouldn't be able to connect to the internet.

Okay, now that you know as much as I do about my network, here's my question:

If I was on box 3, how would I determine the IP address of 4?

Basically I'm running a webserver on box 5 and want to access this webserver on box 3 and other boxes. So that's the end goal. If there's other information there that can help, I'd appreciate it.

Thanks!