I have a machine running Windows server 2012, whose external IPv6 address is "2a01:4f8:190:7449::4". Let's assume it has no external IPv4 address, so the v6 address is the only way of accessing it from outside. But I have services running which are not compatible with IPv6. I want the machine to forward incoming traffic on "2a01:4f8:190:7449::4" to "127.0.0.1", and back. Is this possible?
The technology you're looking for is called NAT64. Note that this will generally run on a router upstream of your IPv4-only service, rather than on the server itself. What to use to implement NAT64, and where in your network to put it, is going to depend on your existing network architecture and the services that need to be accessed using it.
Speaking of those services, if at all possible, they should be made IPv6-capable, or replaced.