Is the 4th position between the middle and bridge pickup on Fender Stratocaster good for Hard rock and Heavy metal or is the output too low? If it is too low would boost or distortion pedal help?

Also is it easier to make a bright sounding pickup sound darker?

2 and 4 are my reggae and country twang spots. IMO hard rock and metal are all bridge humbucker - solos included.

Position 4 will work better for clean tones. If you think of classic U2 songs, that's all position 4 on a Strat.

You can make any pickup sound darker by rolling off the tone pot.
Eh, I think you can get a lot of mileage in solos with the neck pickup, especially more fluid parts like legato and tapping. The warmth really helps smooth it out. I actually think tapping tends to sound better on the neck pickup.