how long will the 4.6 last for

Since you have a good tune, as long as you have the proper safety precautions on place, (i.e. a window switch) you should have no problems with it.
 
the nitrous will increase the wear on your motor, but at 100 your pretty safe, if you're good up to about 150, and then you need motor work to handle that, it should last you a while, as long as you don't beat the crap out of it redlining and juicin up everyday