i think it could be the monsters weapons mh3 has acuatic combat

yes, 4 has a new hub, new monsters, new weapons and systems and exclusive modes, but a few monsters in 3 arent in it

3 being a bit older can feel a little slower than 4 but both are pretty worth playing. If you're new to the old games i think 4u is more beginner friendly.

mh4 has a sand boat