Sure, if you use the default settings. But if you use the drop down and change it to carve up /29 from a /22 you can now have 128 networks. Why would there be a limit of 4? No different that when you carve up an MX template to serve up subnets. The MG devices are standalone NAT devices, in our case, in front of an MX WAN port. It's just another Broadband provider we don't really care what IP ranges it uses because its NAT'ed at the MX. So sure if you care up /26 from /24 you only have 4 networks. But if you carve up /29 from /22 you have 128 networks.
Carve up as /29 (only need three IP address, gateway, primary MX, Secondary MX) and you could have 128 networks