Galera Cluster Setup - Primary and Secondary Site Scenario
"Weighting" was added late in the game, when they realized that a 2-datacenter setup was too vulnerable.  (3 datacenters is resilient, and can use garbd in one of them.)  The example you quote is resilient to any single server, datacenter, or network outage.

As I read the last sentence of the quote, node1 or node2 died but the other three nodes are alive and talking to each other.  That is, there is a Quorum, and the system is still reliable.

However, I agree that the sentence is ambiguous -- It can be read that after the network died, node1 or node 2 died.  This leaves three clumps:  (node1), (node2), (node3,node4), each with a weight of 2.  None should be considered "primary" because none has Quorum.

You bring up garbd, yet it is not in the example??  And where would you put it?

You should not be changing the configuration while the system is hobbled -- you should be fixing the broken components.

The main goal of is to allow a single point of failure -- a single node, the network, a data center.  It would take a really large and complex system to survive two failures.  For example, I think it would require 5 datacenters to survive 2 network failures.

So, focus on a single point of failure.