Can two computers use USB to print to the same printers?

You can turn all 4 printers into network printers using a 4 USB port to ethernet print server like this one at Amazon:

http://www.amazon.com/Keyspan-US-4A-4-Port-Server-Mbps/dp/B0001NA3L2

I've used devices like this for PC networks, but this one says it's also compatible with Mac OS X.2 and up. There are many more of these print servers available, so it's probably a matter of budget for quality.

When I've set these things up on PC's I first had to connect the printers directly to the PC so that I could get the drivers for the printer to install. Then I changed them to network printers. On your Mac's you've already got the drivers working for the different printers, so you won't have to deal with that until you replace a printer with it's new driver.

Of course, for this to work, you will have to have an ethernet network to plug the device into, but you didn't provide any information about your connectivity options.

Neither Mac has to be running for the other one to print.

You can get USB switches that allow you to connect a peripheral to two computers - of course you have to physically switch to the computer you want.

If you have a wireless network, Apple's Airport Extreme and Express have USB sockets to which you can connect a printer which any Mac on the network can access (even those which are connected via wired ethernet).