2. It will be conversing with the router at whatever speed the router can handle. It will converse with the outside world at whatever speed your internet service is supplying.
It also depends on exactly what all the machines are doing at that exact moment. If they are just 'on', they are not sucking up any bandwidth. But...let's say you have 10Mbps service from your ISP, and 4 PC's. If all 4 are downloading something large at the same time...they each would get, in theory, 2.5Mbps.
3. If the other PC's are off, they not using any bandwidth.
4. LED's on the router. Blinking indicates traffic. But that is very misleading. It does not denote constant traffic. Or one of the PC's may be downloading a Windows update while the user isn't using it. Or some application may be making a tiny check for an update.
In other words, don't sweat the bandwidth division among the PC's. Let the router handle it. What you need to be concerned with is having enough bandwidth coming from your provider. 4 PC's on a 1Mbps line will suck.