Alright listen, just a couple of things I want to say first. One is that every community, gaming or otherwise, will have its bad apples. And two is that, there are just rude people in general.
So when someone is trying to act like a gatekeeper with you, saying stuff like "ur not a true Halo fan coz you liek Halo 4," you should think about this: The person who just said that to you is some stranger on the internet who likely has no titles, no respect, and can't even be bothered to spell out their words properly. So why should you care about what they think of you and your opinions? Chances are, if they're uncivil like that, they're just angsty kids and young teens trying to vent their frustrations on something.
And also think about this: You voice your opinion, and someone writes a poorly written insult to you because of that. What will that mean for you later? Will it still bother you a year from then? A month, week, or even a day later? If you realize how little their petty insults mean, it probably won't bother you at all.
It's also a matter of where you voice your opinion. Places like the Youtube comment section will always be full of simply toxic people. But in places like here on Waypoint and the Halo subreddit, people are civil. You're welcome to express whatever opinion you have, as long as you're respectful and present it elegantly. Especially if you support your opinions with logic (or nostalgia. Most people like that, too.)
I've always believed that people think it's just "cool" to hate Halo 4. Though it seems more like it's just cool to hate the latest Halo game. Just take a look at this picture.
There's a surprising amount of truth to it, if you think about it. Halo 4 just gets extra flak because it stands out more than the others with its loadouts and ordnance. But even now, more people are appreciating Halo 4 in retrospect despite all the backlash it has received.