If Stan Lee dies in 2018, should Avengers 4 be dedicated to him?
Well, unfortunately, he has obviously died so there's no "If" there now...
...but that aside, I'd say it's pretty damn likely it will be dedicated to him.
I'd say it's almost unfathomable to imagine there not being a dedication to him in all fairness. If they chose not to then there'd likely be a helluva fuss made by viewers or, hell, even people who haven't seen it. Seems the internet is a pretty place to find people speaking with authority on issues they've no experience of, after all. Joking aside though, I just can't see them not doing that. Like the fourth Avengers film is basically the summit that the entire MCU has been building towards (up to this point, obviously) so if Marvel chose not to acknowledge Stan Lee in that then, publicly, that would be absolutely damning on them as a whole. I mean Marvel, as a company, existed in different forms prior to Stan Lee being there but nobody would dispute the role he played in making them one of the largest entertainment outlets in the world. So ignoring those contributions on such a grand scene would be a huge mistake for them to make. As I say, it's almost unfathomable because it'd be such a ridiculously stupid move to make on Marvel's part.
But, to look at it in a more morbid way, I'd say Marvel "explained" his role within the MCU in an absolutely perfect way. Having him listed as "Watcher Informant" in Guardians of the Galaxy: Vol. 2 basically made him a cosmic/celestial type being within the MCU so, although his cameos haven't ever been important to the storyline of the films he's appeared in nor the MCU as a whole, he's already been shown to be someone who is above the "mortal" realm so it was oddly fitting. I doubt it was their intention for it to be viewed that way in the event of his death but, even so, it certainly works and it gives the idea that, no matter what, he'll always be a part of the MCU, even if we don't actually see him on screen. As I say, it's a pretty morbid way of looking at it but, same time, it's also quite a nice way of looking at it if you want to take it from an in-universe viewpoint.