I hope I combine the eth4 and eth5 ports as an EtherChannel, and allowing them to function as a Layer 2 trunk.
How to combine eth4 and eth5 ports as a Layer 2 trunk
Are you referring to a LAG (link aggregated group)? A trunk is simply a port/cable that is carrying more than one network. Setting up a LAG is what allows you to use two or more physical ports/cables to link two devices together (they can be used to aggregate the bandwidth and/or for redundancy).
If you're going for a LAG, the next practical question is if there is any actual value in doing this (it makes the configuration significantly more complex). It is only necessary if the single port is getting saturated and blocking other activity (for example, if you had a device off the switch that was demanding 1Gbps ineternet (wan) and another one on the switch taht needed to talk to a device connected to eth3 on your main router, for example. It also depends on the details of the hardware you have.
(of course, doing it as an academic exercise is usually a good reason, even if you won't gain any practical benefit aside from knowledge. But be aware that you may not gain any actual throughput unless the right conditions are met.)
Etherchannel is the name Cisco uses for the LAG, so yes.
Actually, since I have two WAN connections, I need a LAN port for downstream traffic, and it would be beneficial to combine both ports into an Ethernet Channel（LAG）. This will increase the available bandwidth.
This is true only if the CPU-to-internal switch chip connection is 2Gbps (or 2x1Gbps) and assuming that the routing engnie can actually operate at >1Gbps.
There is a i7 CUP in the Openwrt box, I think it faster enough to handle LAG.
One use case on my side would be dumb AP, in reality the chance of getting single WiFi stream > 1Gbps is rare, however there could be multiple clients on the same AP and then getting a > 1Gbps uplink trunk makes sense here. Without the need to buy a 2.5G router (the switch needs upgrade as well, and not many managed one) creating two or even more 1G port as trunk is one way out, also now OpenWrt has other packages that can pull internet traffic as well (e.g. torrent). Currently I am using Asus router as pure AP (not OpenWrt compatible) with 2x1G uplink, and torrent client on router itself so my home WiFi client won't feel any degrade when torrent is running.