I'd argue over the terminology (Halo was always a space fantasy), but I agree about the change in tone. Bungie's Halo had a clean, well lit aesthetic, but the story content was really kind of dark. In Halo 4, the renewed war with the Covenant is treated like fun and games.
Regardless of the terminology for the Bungie games, 4 definitely pushes things much further in the fantastic direction.
I didn't think the renewed war was treated as "fun and games", but rather "business as usual". For the Master Chief, the war had only just ended; for the characters not in cryo, their conflict had been renewed for quite a while now.