I noticed the ports 0-3 are close to each other (physically on the device) but port 4 and WAN are separate. Does this mean port 4 is on separate controller or different capabilities?
That link looks like it’s from the previous generation hardware. The Omnia is similar.
You can see the current switch configuration in LuCI under Network > Switch.
The default configuration is: ports: LAN0, LAN1, LAN2, LAN3, CPU(eth0) are in one vlan
and LAN4, CPU(eth2) are in another vlan.
By default, traffic to/from LAN4 to/from the other LAN ports go through software switching (the Linux bridge br-lan).
Why is port 4 on it’s own VLAN by default?
It is in its own VLAN together with the second ethernet interface. This interface gets added to br-lan but may be used as a standalone interface.
I’ve understood how things are configured, but not why. 
The processor has 3 ethernet lines (I’m sure someone from the HW coworkers would explain it is not that simple) and it was decided it would be a waste not to lead them somewhere, so one of them is WAN and the other two go to the switch chip. So the switch chip is divided between the two lines and as the port 4 is on the other side of the middle antenna than the rest of LAN ports, it was simply placed into a VLAN by itself. This way, if you want to have a special VLAN for one device, you can just split the br-lan bridge and don’t have to worry about the switch configuration and the device would be in a visually-separated port.
Other than that, there’s nothing special about that port and you can reconfigure it in any way you like.
So how can i split Port 4 with its on DHCP Server from the rest of my LAN?
You can define separated DHCP configuration for every interface. So all you need to do, is to configure new interface in LUCI.
Isn’t it so, that you have basically 3 GbE connections to the SoC/beckend switch, so basically dedicated 1GbE for WAN, then 1GbE for LAN0-3 and 1GbE for LAN4. So devices in LAN0-3 are sharing gigabit for and LAN4 has dedicated one.
You are free to configure it to your liking…
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Is there a reason for having VLANs in eth1? Does your ISP provide you with a trunk interface with those two VLANs (7 and 8)? If so then maybe you dont want a bridge between eth1.8 and eth2 but routing like you probably have between eth1.7 and ‘lan’? Starting a DHCP server on this bridge might interfere with your ISPs DHCP server (if they use DHCP on VLAN 8).