The web site does exist, but it requires VPN access to the Church's internal network.
Java Stack 4 is the strategic and primary recommendation for building service oriented applications that run on Church networks. However, the artifacts and the project starter service are only available on the internal network. You will need to connect over VPN to access the site and the documentation. That is the reason you are seeing the 404 error. Unlike Java Stack 3, Java Stack 4 is not currently being offered for LDS Tech community projects, so we are providing the link to the internal site while we work on transitioning internal developers away from the LDS Tech site.
We have seen very little need or interest from the LDS Tech community for using Java Stack 4 on community projects, however if you are developing projects for the LDS Tech community in Java, and if you don't have VPN access to the internal network, you can get all the necessary features available to Java Stack 4 by creating a Spring Boot project at:http://start.spring.io/
Java Stack 4 is built upon Spring Boot. We have customized it to facilitate integration with other ICS standards such as WAM, Oracle, and Cloud Foundry. Unless you need access to these features, it is unlikely that Java Stack 4 would give you any value beyond the core Spring Boot platform.