Basically you need a hydraulic motor on each section you want the drive split into, and an extra for the speed encoder.  Plumb all drives in series with an electric valve to let the oil bypass each motor, plus a PWM rate control valve, and something to control it all (ie EZ boom/Insight/Big John/Raven/Midtech/DickeyJohn etc)  Starting out is going to depend on the source of your speed information.  Ours uses a simulated speed signal from the EZguide-500 and I have never seen a problem with any delay on startup.  Actually, you can 'prime' it and get a better start then you would get with a ground drive.  

If you want 4 sections, the bill of materials would be something like this:

5x motor

4x valve

1x flow control 

4x relay 

4x sprocket

4x sprocket and hub

1x EZ guide 500 

1x EZ-Boom 

1x shaft speed sensor of some sort 

A BUNCH of hose and wiring and time to put it all together.