1) Have a look at the code samples
for the Phidgetbridge in the language of your choice (The bridge uses VoltageRatioInput). Once you get an idea of how the example works, you can write code to measure four sensors instead of one. If you want to log the data to a spreadsheet, this article
shows an example of that.
2) The readings of load cells are in mV/V, which means a small voltage proportional to the larger voltage supply of the load cell. In order to convert this number into kg, you need to calibrate using a known weight. For example, measure the mV/V with no load, and then with a known 1kg weight. Then you can use linear interpolation to convert from mV/V to kg. You can use multiple points of calibration to make a more accurate conversion function.
3) You can get the data by either calling the getVoltageRatio method, or by setting up an event handler for VoltageRatioChange. Check the API documentation
for your language for exact naming.
4) I don't know what you mean by zero pressure calibration. Do you mean like a "tare" button on a scale? This would be possible, just measure the current mV/V and create a negative offset that is added to the incoming sensor data.
5) The load cells have no memory on them, so the only way you can tell them apart is which channel on the 1046 they are plugged into. If you use setChannel before opening the VoltageRatioInput object, you can set each object to a specific channel ( 0 to 3).