Some questions:
-Do you want all 4 site's to be reachable using https? (if its only required for 1 site there is no need for SNI or any other extra stuff..(
-Are you testing access to website2 from 'outside' a client or 3/4g phone on the internet? As when testing from the LAN you might actually be accessing the pfSense webgui.?. Though that would still not explain the redirect to website1..
-Is it possible to visit website2 over https on the current IIS configuration? (ignoring the certificate error.?.)
1- VMs seem like a rather big solution to a small problem (+licences)..
2- Haproxy would allow you to configure 4 different certificates one for each domain / ip.
You could even host all 4 sites on 1 external ip, in which case SNI is required to send the right server-certificate back to the client.
3- upgrade IIS of course technically possible, but might require a new windows version (+licence).
4- With the 'old' IIS version i think it might just be easiest to configure the webserver with 4 lan-ip's and change the portforwards to direct traffic to each of those ip's. Then also configure the 4 websites in IIS to bind to those lan-ip's. (you could also try with assigning different ports instead of 443 to the other https sites 1443 2443 3443, and forward traffic there, that would evade the multiple lan-ip requirement, but might lead the site to generate wrong url's containing the port.. something you would need to test.)
I would probably prefer option 4 with multiple lan-ip's or ports, if that isn't possible option 2 or even the combination of both :).