They increased IPC only around 5% with A14. The remaining performance increase was from clockspeeds (gained without increasing power due to 5nm).
Short, wide architectures are historically harder to frequency scale (and given how power vs clocks tapers off at the end of that scale, it's not a bad thing IMO).
4nm isn't shipping until 2022 (and isn't a full node). TSMC says that the 5 to 3nm change will be identical to the 7 to 5nm change (+15% performance or -30% power consumption).
Any changes next year will have to come through pure architecture changes or bigger chips. I'm betting on more modest 5-10% improvements on the low-end and larger 10-20% improvements on a larger chip with a bunch of cache tweaks and higher TDP.
Intel 10nm+ "SuperFin" will probably be fixing the major problems, improving performance, and slightly decreasing sizes for a final architecture much closer to TSMC N7.
I'm thinking that AMD ships their mobile chips with N6 instead of N7 for the density and mild power savings (it's supposedly a minor change and the mobile design is a separate chip anyway). Late next year we should be seeing Zen 4 on 5nm. That should be an interesting situation and will help resolve any questions of process vs architecture.