Indeed, a major memory problem, running out of space. What are your detail settings, including micropoly detail, AA, GI, cloud and atmosphere samples, and soft shadow samples on light(s)?
I also want to mention that if you were customizing the Subdiv Cache sizes in previous versions of Terragen, you should not do so with TG4. We've put that setting into Subdiv Settings, but I think it may remain customized if you load an older project. So if you're rendering something you originally created in TG3 and you customized the subdiv cache size, returning that to auto may help reduce memory use.