Definitely a great first installment from 343, though this is coming from someone that is single player campaign focussed. I played Halo 1 2 & in a row prior to Halo 4 so feel my 1st month of Halo 4 can be compared with thee previous games with a fair degree of freshness.
For me the Halo main series runs as follows:
1. Halo CE - original and best, no matter how many times I play through I have a blast and still have new encounters. Never got the hate for the Library etc.
2. Halo 4 - I had a blast with Halo 4, and to be honest I think there are 2 main reasons for that; it seems clear to me that 343 went back to CE for their main inspiration which may explain some of the 'smaller' scale levels compared to 3. Additionally Halo 4 had NO BRUTES!!! I fucking detest those monkeys, never enjoyed fighting them, and found the Prometheans coupled with the other Covenant staples in Halo 4 to be a vast improvement.
Oh and I don't care what people say, but Halo 4 (at least on my TV) was a MASSIVE graphical upgrade to previous games. However, it gets minus points for dissapearing weapons! That is a major PITA on Legendary.
3. Halo 3 - Enoyed it much more than the time I had with Halo 2 in my replay run. Loved Campaign Scoring and it's something I miss in Halo 4. That said, Brutes suck, and Halo 3 is mostly Bruteville.
4. Halo 2 - Really surprised by how much I disliked it sandwiched in between CE and Halo 3 during my replay run. It was so cheap in places (Sniper Alley, Cairo Station Cargo Bay!) and I generally didn't have anywhere near as much 'fun' as with CE, and subsequently 3 and 4. Narrative execution was poor as well. That said, playing as the Arbiter was a plus point.
This order could potentially change on subsequent playthrough's of Halo 4, and I shoudl also qualify that I've alwasy been a predominantly solo campaigner, and I could easily see people who loved co-op in Halo 3 having more issues with the scale than I did.
Halo 4 got back that alien otherworldly feeling that CE nailed. Really enjoyed Davidge's soundtrack too.
Oh, and Frankie if you are reading, ditch the QTEs for Halo 5 and keep weapons persistent mmkay? Thankee!