The 9's have 2 enet ports, that should let you tie all 4 together in a daisy chain formation.
I understand you are adding one HDS 9 at the bow and one at the console. Get two 6 foot Lowrance Ethernet cables. (It's the shortest they make.) Install the two HDS 9 units, moving the existing Ethernet connection from the old unit to the HDS 9 at each location. Then, use the 6' Ethernet cables to connect from each HDS 9 to the old unit where the Ethernet cable had been removed - you're done.
What I described saves $ on buying cables and puts the StructureScan box in the middle of the network, which is desirable.