Probably why neither of those items lasted very long. The first 5 P4a's came with both, and eventually the second group of 5 P4b's came with only sky-lining, and both ended up off shortly after. All the skylining does is add semi streamlining and prevent dirt from getting on stuff, but in the end it just adds more work to the maintenance in those areas. As for the deflectors, they added them super tight to the smokebox so they didnt hinder the engineer/fireman's view, but with little room for draft, they didn't work very well.
One thing I noticed is that the P4 skyline casing doesn't go over the entire engine, unlike a few other skyline casing engines, like this
And this
And also this