The standard r2 U2 r2 Uw2 r2 u2 algorithm looks intimidating, but it is highly symmetrical. Here is how to memorize it in 30 seconds:
PLL parity occurs in about 50% of 4x4 solves and is characterized by having two pieces swapped that cannot be fixed by standard 3x3 algorithms. 4x4 pll parity algorithms pdf