When you solve the 4x4 Rubik's cube there are two types of parity you can get. Here are the solutions:

Fix dredge parity: r' U2 l F2 l' F2 / r2 U2 r U2 r' U2 / F2 r2 F2
Fix two adjacent corners: r2 U2 r2 / (Uu)2 r2 u2

Key
Left to right: L, l, r, R
Top to bottom: U, u, b, B

I made a video of how to solve dredge and corner parity.