you could add a second route with a higher metric so when the ping fails for track 100 and the route is removed from the routing table, the second route takes over.
sla monitor 100
type echo protocol ipIcmpEcho 3.3.3.3 interface Outside
frequency 5
sla monitor schedule 100 life forever start-time now
track 100 rtr 100 reachability
ip route 1.1.1.1 255.255.255.255 2.2.2.2 track 100
ip route 1.1.1.1 255.255.255.255 4.4.4.4 254
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