I've been watching it on ABC's website which probably doesn't factor into those ratings. Not only that, but plenty of people watched s1-3 on Netflix. Ratings don't mean as much nowadays.

Agents of SHIELD needs to lose ALL the Inhuman garbage and shift in to a pure spy thriller show. Let the Royal Family do Inhumans right and be the Inhumans from the glory days and not some X-Men wannabes.

Infinity War How much of Infinity War will be on Earth?
If portions and events are on Earth, then certainly SHIELD would be involved in the fight, in coordination with the Avengers and other powered people. If the entire build-up is in some other realm of the Galaxy, then probably not.
For instance, Winter Soldier, Ultron and Civil War all had some sort of involvement with SHIELD since they happened on Earth, but Guardians, not so much. (Though I'd really like to see Quill come to Earth. Who else but Coulson would be most qualified to go meet the Milano? )
On the other hand, I wouldn't be too quick to tie AoS into the movies. Season one was very tied into Winter Soldier, since it was the destruction of SHIELD, but since then they've been doing a pretty good job taking the show in its own direction. Ghost Rider seems pretty successful, why not bring Robbie back for a while in S5. Once the Inhumans series ends, a story arc in AoS could involve those characters. Quinn and Magnetron are still out there somewhere. So is Deathlok. The writers must know the fans have been asking about them
And I would like the Team to cross paths with Hunter and Mockingbird every once in a while, perhaps even a short story arc. I don't know about them returning to the show, but considering they are in the same profession, I could see them showing up as allies every now and then

Considering AoS is already in syndication, I'm hoping it was sold with a condition there'd be more shows produced.
(I was flipping channels the other day and came across Ward, Garrett and the team working together before the HYDRA reveal. Man, you'd never know they were about to turn evil. After I get the Season 3 Blu-Rays, I'm going to have to binge all three seasons)

Well, according to this article, season 5 might happen.
https://www.comicbookmovie.com/tv/marvel/agents_of_shield/abc-president-very-bullish-on-an-agents-of-shield-a148068

I'm a bit out of the loop so is there not going to be Season 5? If not quite a blow for Marvel on tv as no series on air if that's the case.
Is there not going to be 2 Infinity War movies now? If not, can't see it working as too much to tell in 1 movie and the whole franchise has been building to this moment so could be a right turkey.

I know. But it still sounds like it's going to be a 2 parter.

There is no longer "Part 2." Infinity War is just Infinity War.

I want the show to continue just to see it tie into infinity war. But I don't see how they can do a cross over until Part 2 comes out in 2019. They wouldn't want the show to give anything away before the movie comes out.

I don't think there will be a 5th season, but if it happens, I would love to see Hunter and Mockingbird back.

But not as much as L+SD which is what I said.

Ratings are dropping. DVR ratings this season are down as well.

Feige literally said about 3-4 months ago (in regard to, What about SHIELD? What about Netflix?) "Everyone's in it whoops, is that a spoiler?"

With ratings dropping,
Shifting to DVR more than true dropping.
I'm betting Perlmutter has come around to letting the Agents of SHIELD actually appear in the movie in some capacity
I have never heard one word indicating that Permutter has done anything like that and frankly don't believe you.
but how about on the show, next season, leading up and into Infinity War?
If it's going to happen it has to be mid April up to late July and that's if none of the cast has other work scheduled for that period. After that July date they will be shooting S5 covering the rest of the shoot for AV 4.
Should Hunter come back? Should Mockingbird?
No.