In the trailer Black Diablos, Plum Diamyo Hermituar, and Hermituar have returned in mh4u.
In the trailer Dundroma town is an returning town.
Steve shares a relation with bats and birds.
Steve has the ability to fling spikes and can fling them at enemies.
Steve will migrate to different areas.
Many desert monsters are found else where in mh4u, unlike many monster hunter games.
Hermituars can now be found away from cool areas or areas with water in the desert, unlike mhfu.
Me just guessing
There might be a new weapon, the poleaxe or pole arm either one, and may be only used by the town guards of Dundroma.
Steve might inflict a new status as in the trailer a hunter in zinogre armor was taking damage when trying to move after getting hit by the spikes, giving off a effect that looked like spikes falling off.
Amatsu might be returning, and Rusted kushy might now be fightable.
There might be an new elder dragon. as in the trailer everyone was looking up at an mysterious figure that turned the sky red and rainy. (Sounds like the weather you fight amatsu in, possibly meaning Amatsu will be fightable at the Town Fortress.)
Steve might appear in both Unknown Sea of Trees and Ancestrial Steppe.
Water fighting might return in future games after seeing an ocean/oasis in the distance with bright blue water, in a caved in area in the desert. I think it looks to much like an ocean.
Steve could be driven away instead of migrating because of an unknown new elderdragon.
Not many monster hunter games left? Maybe, its just many old monsters and old places have been returning. Maybe we will only get to mh6u. :(
Many elderdragons could return such as Loa shan and Yama stukami and even Shen goaren, after carapaceons have been hinted to be ingame.