1. Replace all manual valves with Solenoid valves(150 meters distance for the 4th valve). Do I need normally Open or Closed? Any specific product?
Most valves are normally closed (for safety reasons so if power is lost it won’t leave the water flowing).
2. Use OSPi to configure the schedule for 1 hr per valve, accommodate the frequent power outage(not sure how its possible).
As Dan said, you will likely need a backup power supply to keep Pi running. Ultimately you probably want to look at battery-powered controllers with latching solenoid valves — these can be powered by AA batteries and do not rely on power adapter. You can take a look at the OSBee Arduino Shield to get an idea of these types of valves:
http://rayshobby.net/cart/osbeeshield
3. Use OSPi to switch ON the 3 phase Starter automatically, aligning with the OSPi’s 1 hr schedule for each valve (total ON time will be 4 hrs).
If I understand this correctly, this is what a master station can achieve, and master station is already supported in OSPi.
4. Monitor the power line voltage for all three phases, monitor current (amps) – for dry run deduction and log the values into RPi. Use the OSPi’s analog pins.. but not sure what product should I look for to read this info.
This will be a bit tricky because it involves current sensing on the mains. I don’t know much about this.
5. Use Soil moisture sensor and let OSPi to do the rest automagically 🙂
I have been doing research on soil sensor recently. I just wrote a blog post about reverse engineering an off-the-shelf wireless sensor, which may be useful for your application:
http://rayshobby.net/?p=9413