I'd recommend talking to a workshop about your "future works" before you go and spend money on things you may not require You really need to get some plan of what your looking to achieve before you just randomly buy parts. Sounds to me like you have money burning a hole in your pocket.
That plus the fact I don't know any skyline running a 4" system, 1 they wont fit of the turbo, 2 I don't see it being necessary. I also run a BATMBL 3" front/dump and it's tops. With a special cat and a decent 3" system you'll be more than fine, ask any tuner.