7.x-4.0 contains significant api changes. Although reports written in 7.x-3.x versions should work the same in 7.x-4.0, changes to the security model have been made.
The report editing user interface has been completely redesigned
Note that the fields tab has been replaced by clicking on the gear icons next to the first occurrence of each field. Also note that you can now use the template editors to make changes to each block. You can add and remove data sections to your report as well. Feedback would be appreciated via the issue queue. Also, The edit and translate tabs have been changed to iconic links that will keep report parameter context to simplify the editing process.