For PA tax purposes, your basis is the sum of all your contributions into the 401k plan (do NOT count employer contributions or earnings) minus the sum of all distributions taken in previous years.
Essentially, you get your contributions back before any part of your withdrawals are taxed by PA in the case of an early distribution (box 7 coded 1 or 2).
Your contributions to a 401K would be coded "D" in box 12 of your W2.
