Thank you very much for your reply.
I've managed to get it to work - thanks for your help. A couple of points:
• I needed also to add a rule specifically to allow DNS traffic from the DNS Resolver in the firewall across the 4G WAN, otherwise DNS doesn't work (because it doesn't hit on the LAN rule)
• In addition to changing the gateway manually (which is fine), I also need to tweak the DNS Resolver setting so that outbound requests go across the 4G WAN and not the normal WAN. Not sure if there's a way around that? If I enable both outgoing interfaces in DNS Resolver, then it seems to distribute DNS traffic even when the gateway doesn't need to failover.