4.3 isn’t officially out yet, only the preview, when it’s released (very soon probably) the UE4 Speedtree license will be out for purchase as well.
If you see people using it already it’s because some of us got early access.
Also to answer some earlier question, if I recall correctly you can only import trees from speedtree 6 and up into the new modeller.
Any Speedtrees not made with the new Ue4 modeller won’t be able to be imported into UE4. Your best bet would probably be to export as a static mesh and then set up all LOD and that stuff by hand if you intend to use old trees, which doesn’t seems worth it, better off just making new ones. As long as you still have 2D assets it shouldn’t take that long, the modeller is very easy to work in now compared to the older version.
I think better idea is to model a new tree from the scratch. I know how to model a full tree in 3ds max but it’s needs much time. Speedtree does within 5 minutes while modeling needs 2 or 3 hours for a tree.
However I think I have to model tree by hand for now.