In this scenario, both routers are advertising the 4.15.227.0/25 subnet, only the secondary is advertising with an artificially more distant path, this wouldn't be chosen unless the primary is down.
The inside, when using OSPF, for example, the secondary advertises the default gateway with a less favorable metric than the primary, thus on your inside switches, you end up with two default routes, but only the best one would be used.
In the case that the primary pfsense goes down, then the secondary's routes become the only remaining routes on the ISP and Internally, and it keeps on working.
You could also put a link between the two boxes to route traffic over it in the event that either just the inside or the outside link goes down on the primary, then the traffic would flow through the secondary on the cross-over link, or if your switches support multi chassis LAGG you could add redundancy that way too.
Because pfSense is a stateful firewall, under certain circumstances, the session would drop, but for web traffic it wouldn't be noticeable for the most part.