Raspberry Pi 4 has only one Ethernet, and to be used as a router you will need an USB Ethernet dongle, which IMHO is less than ideal. This way I would rather go with a NanoPi R4S. It is cheaper and much better than the Raspberry Pi 4 to be used as a router (4GB RAM, 6 core CPU and two Gigabit Ethernet ports).
I actually used the AX6S for a while as a main router, and now I've replaced it with a Nano PI R4S. The AX6S has support to hardware NAT and can also achieve Gigabit WAN speeds without SQM.
Below is a CPU (coremark) benchmark I did just for reference (I do not own a Pi4) I recently posted in the NanoPi R4S topic - notice its is a single-core benchmark:
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Device SoC Cores Clock Coremark 1.0
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Odroid C2 Amlogic S905 4xA53 1.50 GHz 4997.917534
Redmi AX6S MediaTek MT7622BV 2xA53 1.35 GHz 4681.282671
NanoPI R4S Rockchip RK3399 4xA53+2xA72 1.4/1.8 GHz 4990.435000 (1)-A53
NanoPI R4S Rockchip RK3399 4xA53+2xA72 1.4/1.8 GHz 10535.742506 (2)-A72
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(1) A53 core: taskset 0x000000001 coremark
(2) A75 core: taskset 0x000000010 coremark
Below is also a WAN speed test I did with AX6S that I had posted some time ago in this router's topic:
