Transforming volts to bits
Updated answer:

It looks like you only want to detect the presence of voltage, not actually measure the voltage. If this is true, you can avoid a micro and use a USB to GPIO chip (such as the CP2103: http://www.silabs.com/Support%20Documents/TechnicalDocs/cp2103.pdf). The chip I listed above is actually a USB-to-GPIO chip which also has 4 GPIO inputs. You would be able to program these (from your host) as inputs and then read the 0 or 1 state of each pin.

Old answer:

If you really want to build this, what you need is a microcontroller with an analog-to-digital converter and a USB interface. You will need code running on the micro to read the value from the A/D and make it available over the the USB interface.

Examples of suitable micros would include the C8051F321 or SiM3U134, both from Silicon Labs. There are many others, from many other manufacturers.