Whatever web framework you're using (probably some variation of Spring if you're using Java) should have built in templating which should be more than sufficient to build an MVP/Proof of concept imageboard. I doubt 4chan is running any kind of SPA either.

Then if your project is successful or you want to stick with it, you can build your frontend javascript app as a phase 2. In most webframeworks you'll do largely the same query/controller work to get the data for both a JSON API and a templated web page. The API will just map that data to JSON while the templated page will map the values into an HTML response.