I was looking for more information online and found out that you’re absolutely right. The wall heater is actually two separate 2000W heaters wired in series.
I’d have to slightly alter the wiring to install a Sonoff POW onto each heating element, so, if I do this, I’ll talk to an electrician / contractor / any other inspectors to check my work before I turn it on. I’ll have to monitor the Sonoffs for a week or so to ensure they’re from a good batch too. But that should do it!
Other design/safety notes: The heater’s internals/rheostat are also already self-enclosed in a metal box with good air flow for safety, so the sonoffs can just join the encased internals. Also, importantly, I realized that I may need to test wiring the sonoff behind the rheostat instead of replacing the rheostat entirely (and get another rheostat to install on the heating element that doesn’t have one since both are currently wired in series and under the control of just one rheostat). This way, I can check to see if the rheostat prevents the heater from over-heating itself. I wouldn’t want my HASS server to lose wifi connection for some reason and forget to regulate the Sonoffs controlling the heater.
EDIT: After more consideration, not modifying the hardware or using dodgy Sonoff POW’s seems like the more responsible route even if it is technically feasible. Installing the Z-wave 40A switch on the main line seems like a lot safer option from a liability standpoint. The Z-wave switch and a Z-wave dongle would be pricey, but I think it would be worth the peace of mind. And, again, I’d have it inspected before turning it on if I went that route.
Thank you guys