I remember from 3rd ed there was a rule for half of the hits for the assist were added as a dice pool modifier to the main roll. Does anything like this still exist?

Yea it's called teamwork and all the hits are added as a dice pool modifier to the main roll.

This dice pool modifier can't be greater than the primary character's skill though, from what I remember.

For defensive combat assist rolls, look under "suppressive fire".