Sorry not sure if i understood your requirement corrctly.
WAN Set to Bridge Mode
LAN 1 -> Connected to
Raspberry Pi in which I will enter PPPoE Username password.
Now I have a LAN IP 192.168.xx and an IP from PPPoE on the
Pi.
Answer: Yes, you can do it with Bridge option
LAN 2 - 4 -> Connected to
Laptop/
TV other client devices. 192.168.xx
Set Raspberry Pi as the gateway
Now will the devices in LAN 2-4 be able to access internet by going through Raspberry Pi?
Trying to answer this. For eg. you have created WAN profile say WAN0 as bridge mode. And your secondary
router is connected in LAN 1, or 2,3,4 as bridge. You can get internet only on secondary router thru bridge mode. Other LAN ports in Dasan will not get internet thru any of ports until you disable the bridge mode and return to Dasan PPPOe mode.
Dasan restrict the creation of multiple PPP0e session what we have seen in other router. Its restrict with VLAN ID. So you can create any WAN profile with just single VLAN ID 100.
i have less knowledge about Rasberry pi. Please ping me on telegram, will try to clarify .