I have 2 bridges

Bridge 1:

wlan1
ether2

Bridge 2:

ether3 (internet comes through here)
ether4 (where i connect pc to configure)

I want to share the internet connection from ether3 to wlan1, how do I do this?

Why do you want to use 2 bridges? There should be no need for this, except that the "ETHR4 PC" is in the same L2 net than
the "internet box". But most boxes are managed via L3/IP so thus no need for this and you just have ETH3 as Internet input (WAN).

If you want to keep your setup, you will need to add a route under IP to tell the router where the internet is coming in.
Example: Say you Internet GW in bridge 2 is 192.168.100.1 then you need to add route: 0.0.0.0 GW=192.168.100.1

You will also need to adjust firewall rules as the default rules are not for such a setup.

Post your config as your design is not usual.

Typically one does not need to assign the internet etherport to any bridge.
One simply assigns the dhcp client as appropriate and ensures WAN interfaces and interfaces members and firewall rules etc all jive.

All the other wlans and etherports can be on the same bridge.
If you need to separate folks, then you can either do it by etherport or vlans .