I'll try to help here.

The LGC 4000 antenna is a NMEA 2000 device, which means you are connected using a NMEA backbone network with T connectors. In order to use it on the HDS 8, you'll need to connect another T connector to the NMEA backbone and run a NMEA cable from the T to the HDS 8. In addition, I'd recommend that you connect another Lowrance Ethernet cable back to the LSS box from the HDS 8.

You essentially have 2 NETWORKS: Ethernet for sharing Sonar 2d/StructureScan data and NMEA which will share the Antenna.

DITTO for the Point 1~

Here's a video of a Point 1 Antenna Install, which is the same as the LGC 4000 as far as set up instructions.

https://www.youtube.com/watch?v=SXdk8swHTiU

If you have the $$ I would recommend that you sell the 4000 and get a PT1 instead. The reason is the POINT 1 has a compass ( Directional ) so whatever direction the boat is facing will be the same direction that the HDS units are showing, plus the PT1 is 2x faster picking up the satellite signal. The PT 1 allows you to waypoint a spot ( brush pile/bridge/ structure ) and be able to quickly find it while looking at the HDS. This eliminates the need for marker buoys to locate the spot.

Hope this helps,
Jeff