How is your local lan connected to the remote lan? Based upon your description you have 3 networks to worry about. 192.168.100.0 172.16.4.0 and 0.0.0.0.
Assuming that it is a physical connection you have to have a router with 3 interfaces to route this traffic.
From what I saw on the Watchgaurd site, all of the appliances have at 6 interfaces, so you should have one connected to the Internet one to the Inside (local lan) and one to your remote site.
If it is a logical interface where you are connecting through the Internet and using an IPSEC tunnel, then you just route to the logical interface.
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