You are pretty limited with a 4a limit. Don't worry about leg loss and temp, do tell it how long your legs are going to be as that can affect how many wraps you are going to need.
First start with the ohms tool to see what you should be shooting for in resistance. Lock the current (amps) and play with the others. At 4v you should build a 1.1 ohm resistance coil. 15w may be higher than your pv's limit though... maybe not. Again, play around with the variables to see what gets you to the correct amp, watt and resistance combo.
Then move to the coil tool to find out how many wraps you need to do on your given bit size and wire gauge.
I came up with 4 wraps single coil or 8 wraps dual coil on a (approximately) 3/32 bit.. This all may be above the limits of your pv but worth a little messing around with to see if you can get it to fire up correctly and not take forever to heat the wire up
