Make Tier 4 server nodes harder to attack (easier to defend) by upping the math or possibly add alternate paths from the Player's base to the Tier 3 server nodes.
We have found out that soon after the 6th in players start moving towards other player's bases. Their success rate is pretty standard; however, I think there should be a "defenders advantage" on the Tier 4 server nodes to keep the game in motion. Another option is to add one to three alternate paths from the Player's base to the Tier 3 server nodes. This might alter the mechanics in a way that players will not secure their Tier 4 server nodes; however, we can maybe force them to take those Tier 4 nodes first and then open the paths to the Tier 3 when they acquire those server nodes.