You would lagg the connections, depending on switch maker terms might be etherchannel, or port channel or teamed. All pretty much same term for doing the same thing binding connections together for loadsharing.
This provides you with multiple paths for a failover issue while also allowing you to leverage more bandwidth between the switches for loadsharing. In a typical setup you might even connect switch 2 to 3 to allow for another path if your homerun to your main switch went down you would have another path to the switch via the connect. You would leverage spanning tree (stp) to block that connection so you don't have a loop unless the home run connection to the main switch when down. That connection would then come up in forwarding vs blocking.
So for example is that fiber connection only 1 gig? If just using it as failover with 1 connection only being used all your devices on switch 2 for example are limited to this 1 gig uplink to anything on switch 1 or switch 3 or internet. Not sure where your servers are for example.
Typically in a case with location that has need of that many ports you would have way more than just 2 network segments/vlans. Without understanding your environment and amount of data flow between devices and where they are connected its hard to say what your best setup would be.
How are you leveraging those 4 wan connections? How fat are those pipes?
What other types of devices do do you have? Servers, printers? Voip phones?
In a typical smb setup you might see say 5 vlans for sure.. Depends on what you want to isolate for security, what your using as your routing for intervlan.. How much intervlan traffic your going to have, compared to security concerns. For example you might just have a data vlan and you would put all your servers/printer/users/networking infrastructure management all on this data vlan. If you have phones this normally would be on a voice vlan, and then your wifi normally atleast 2 1 for internal use of known users and devices that need access to your other stuff, and then just a guest that has just internet, etc.
Typical you might have
infrastructure
Data
users
voice
wifi
wifi-guest
All as different vlans. With data possible broke up even more into servers/printers/production/etc/dmz and then depending on the number of users or different types of users you might have multiple user vlans. This might be office users, engineers, management, sales, finance, kiosks or plant floor..
Shoot in my home I have 7 different segments and vlans for gosh sake ;) If anything that number would just go up.