Here is my circuit for controlling 4 solenoid with 4 GPIO pins of the Arduino.
An external power supply will power the solenoid and not the Arduino.
I'm using a low level Mosfet.
220R resistor for some protection and 10k pull down resistor(?)
Any suggestion / comments on the attached schematics?
You seem to have four separate modules, each with its own power jack and polarity protection diode. Is that right?
The circuit itself is fine.
Yes, exactly. I made it because the solenoids might be placed far away from each other so 1 power supply will not be practice.
If your plug-in power source has center contact as positive then the input connector is wired backwards.
Yes. Missed that before! 
Centre positive is the agreed standard. There have in the past been a few rogue manufacturers (Panasonic).
Seems like a good application to add opto isolation.
Can get 4 of them in one IC ... here's a popular choice.
Every solenoid should be placed as a standalone or max in a group of two so 4 onto isolation in 1 IC is not good for my application. If adding a opto solution how shall I place it?
as the following?
Looks good.
You don't need R5 because the opto's CTR limits this.
The grounds need to be from separate (isolated) power sources.
What do you mean by that?
Shall I not connect the arduino ground and the 12V ext power supply ground together?
In the optocoupled solution no, in the non-optocoupled solution yes.
That's correct.
Otherwise, isolation is lost and the optos wouldn't be needed.
The optos are useful to greatly reduce interference and improve noise immunity, which is good when longer distance wiring is used.
Sounds like you already have a separate power supply and will have the solenoids some distance away from the Arduino (this is good).
And more specifically, it is important that you have the optocoupler in the assembly with the FET, not remote from it, but that both input wires to the optocoupler - the control wire and the ground - run together as a pair from the controlling device to the solenoid driver. So the grounds will be quite separately connected.
the control wire meaning the arduino gpio output?
I thought I should not connected the ground from the controlling device (aka arduino). What do you mean by "run together as a pair"
Yes
Correct, if you mean not to connect the Arduino ground to the solenoid power ground. But you will need to connect the Arduino ground to the cathode side of the opto-isolator.
You would probably use 2-core cable to keep the Arduino ground wire and the wire from the gpio pin together at all points. This will help reduce the effect of interference, because both cores inside the cable will pick up the same interference.
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Assuming using this 12V solenoid:
if the rated current is 0.8A - for controlling 4 Solenoid with the same power supply the power supply should be rated for 12V 3.2A? how can I know what power supply I should use if I want to power all the 4 solenoids from the same power supply?
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