If you google LED calculator then it will work out the lot for you.
I would set source to 3.6V and set led voltage to 0.5. You must use a resistor so the maximum voltage it can give you is 3.6V.
It works out at 120 Ohms
No, in a parallel circuit you get equal voltage across each branch, its the current that divides between branches.
So I assume if you wanted to run 4 LED's in parallel with 3.6V and 50mA each, the source would have to be 3.6V and 200mA. Don't take my word for it though, I'm crap with electronics.