Wt 4 is our update to C++11. There are many small differences, but one major one is the change from raw pointers everywhere to using unique_ptr for ownership. Check the Wt 4 post for more info: https://www.webtoolkit.eu/wt/news/2017/09/19/wt_4_0_0_is_here_
I don't recommend that you start new application development with Wt 3, since we will stop supporting it at some time.