Halo 1-3?

Correct. The numbering is from the games. Along with Nightfall and Fall of Reach there's Halo Legends which is a series of short animated films set in the Halo universe, like The Animatrix if you've ever seen that.

VincentVegaFFF already answered your question wonderfully, but I just wanted to say it's really cool of you (a non Halo-gamer) to watch and be interested in both FUD and Nightfall, and possibly even Fall of Reach. Hopefully you've liked what you've seen of the Halo universe so far? :)

It's not a silly question if you haven't played any of the games, not everybody has got down with some Halo. I wouldn't expect those people to watch FUD or Nightfall though, it's not how I'd introduce anybody to the franchise. Did you enjoy them? I figured they were probably only cool to people who've played the games, they're just sort of supplemental material and nowhere near as good as the story of the game franchise. And I imagine they could be a little confusing. Part of it takes place right before the events of Halo 4 and part of it takes place 30 something years earlier. At the end of Halo 3 The Chief and his Covenant buddy The Arbiter escape The Ark (a main hub that can fire the other Halos and constructs new ones) in the UNSC Forward Unto Dawn. But as it tried to enter slipspace it was sheared in half with The Arbiter making it back to Earth in the front half while the Chief and Cortana in the back half were stranded in unknown space and presumed lost. So Chief goes into cryo sleep and tells Cortana to wake him when he's needed. Meanwhile she starts to go rampant (AI's only have a lifespan of 7 years before they go insane), which is why in the beginning she's acting all crazy.

I think the only reason for them putting "Halo 4" in the title was because it was a tie-in/promotion released just before the launch of the game. It doesn't really have THAT much to do with the actual events of the game Halo 4, other than introducing those of us who don't read the Halo novels to Lasky. He hadn't appeared in any previous Halo game but has been a character in 4 and 5. He's a pretty cool supporting character, and just like the younger version of himself he's always disobeying orders. Kelly and Fred don't show up in the games until Halo 5.

I didn't think it was that great but as somebody who had been playing the games for over a decade (in 2012) it was pretty cool/surreal to see Spartans and Covenant in live action. I was a little thrown off by Chief's voice (they didn't use the regular voice actor's voice) but he was still awesome. Especially when he blocked that needle rifle shot from hitting that kid.

"Dan Marino should die of gonorrhea and rot in hell. Would you like a cookie son?"