Is there any way to disable WiFi if the Internet connection failover to 4G connection?
I do not want to let users to use 4G Data and run out of the credit if primary connection is off for some reason.

Does your router have the "Applications"/"Smart Action" option as you should be able to use that to set a firewall rule or route policy when the WAN connection disconnects, then put it back when it reconnects.

For example, product AP912C has a feature "WiFi auto on/off"

The configuration is to define an IP address that is 'pinged' and if the address doesn't respond, then the WiFi switches off.

It may be possible to use the ISP's WAN gateway IP to identify failover and hence to disable WiFi.

I don't know how widely "WiFi auto on/off" is implemented across products.

I do not have any of these.
I created a filter in Firewall:
Firewall>Filter Setup> Set1:
In the "Direction" I removed all of the interfaces and left only WAN5 (4G USB) so when WAN5 is working the whole traffic is completely blocked.
But I need to exclude ssh. Is there any way to do it?

Yes.

Have the “BLOCK ALL” rule lower down in your list of rules in that set, they work in order so put that “BLOCK ALL” rule last; at least lower down than the rule you’re about to create.

Now create a new rule above that allows the SSH ‘Service Type’ to ‘Pass Immediately’.

Because the rules work in order, if you block all first, the traffic will never reach the “Pass Immediately” rule it’ll simply get dropped, so place it before the block all.
Everything else that isn’t an SSH service type will be blocked; unless you add another rule before the block all again, or as many as your router will allow.

(Note: If you start expanding into another “Set#” don’t forget to choose that next set# in the lower right corner of the set page, otherwise it will never progress into the following set.)