Except in extreme circumstances, the angle of a beam will no longer change as the spacing expands and contracts.
Sibelius is now able to correctly determine the stem direction of all notes under a beam, enabling improvements in note spacing (in particular, in cases where Sibelius does not allow enough space because it expects the stem is pointing in the opposite direction to that in which it would point if it were unbeamed), ties (which no longer appear to flip over page breaks as you drag the score around), and magnetic slurs (which no longer flip over page breaks).
Start repeat barlines at the start of the system are now spaced correctly and do not collide with key signatures etc.
It's now possible to drag articulations above rests independently of the rests themselves (this does not apply to bar rests).
Arrange no longer adds dynamics to staves to which it adds no notes.
The following problems from earlier versions of Sibelius have been fixed in Sibelius 4: Arrange