When I used the free SDK of UE3, I was able to use lots of free assets such as : a waterfall, wind moveable trees, incredible floaty effects, and various modes of transport etc. Can I do the same in UE4 for free without having to first purchasing things (eg the waterfall ?)
Yes, just enable starter content when creating a new project.
Yes, also, All the Assets that are available for free from any example projects in UE4 (On the Learn Tab of the launcher) are allowed to be used in any Unreal Engine 4 project, commercial or otherwise.