I’m assuming that you’re using an older version of the SpeedTree modeler? Because, the Unreal Engine 4 version has yet to be released. Thus, you’re likely using trees exported from SpeedTree 7, which aren’t compatible directly with UE4.
However, if I recall correctly, the SpeedTree folks mentioned in the Twitch stream that trees created with SpeedTree 7 would be able to be taken into the new modeler, then exported from there. Then, they’ll supposedly work. Though, I’m not sure if that’s changed since.
For the time being though, you’ll likely have to wait on the release: http://www.speedtree.com/ue4/subscription.php - hopefully it’s soon, I’m awaiting on it also! 