I would like a config where the modem hands out :

• a public ip to internal wan dhcp client

• a public ip to the settopbox

eth3 & eth4 & wlan are a 10.0.0.0/24 subnet

Can someone help me , i am poor in bridges / vlan / switch

Create a WAN-Bridge add ports 1 and 2 to it. Likely change anything in your config that references eth1(DHCP Client, IP address, firewall rules, NAT Rules, etc) to WAN-Bridge. With newer default configurations you may also need to add the WAN-Bridge to a WAN address list (I forget the exact name of the address list).

If you need help, post the results of
export hide-sensitive

Do you have a block of IPs from your ISP??
If yes, then you are good to go with a setup.
If not, then what is the purpose of the setbox??

Yes i get multiple ips from isp.
Settopbox is required by isp and needs a public ip

If I make a wan bridge , will it also use the switch chip?

Only one bridge can be HW offloaded (i.e. can use switch chip), other bridges will be software-only. When you create two bridges over ports of same switch chip, you can actually select which one should be HW offliaded by setting hw=no on all ports members of bridge where HW offload should not be active.

Performance degradation (or rather CPU load increase) only affects the traffic switched between ports of such bridge (e.g. WAN-bridge), it doesn't change the way routed traffic (e.g. between LAN and WAN) is handled, that part is never HW offloaded.