This is a hard plastic firewall grommet for 4 AWG wire. It has a ip68 rating to help keep water that may splash onto the wire from entering the cab.

Fits 4 AWG with flex loom
Does NOT fit #FAM brand 4awg with flex loom, only without flex. If you want #FAM 4awg then use the larger size up from this. 
10-14mm inner diameter
IP68