Here is a comparison of the Define R3 and R4, side by side – the R4 is the larger, black case and the R3 is in white.
When the front panels are lined up like so…
The rears meet like so:
Same length! As for the width:
As you can see the R4 is substantially wider – it also replaces the grille and twin rubber grommets for water cooling with a vertical PCI expansion slot, which is arguably more useful. The rear exhaust fan mounting has gone from a 120mm-mount to a 140mm mount – 120mm fans can also fit, mind. The front intake and top intake/exhaust fan mounting points are also 140mm-compatible now as well on the R4 (again, 120mm still fits).
As you can see the R4 is slightly higher, too.
More comparisons to come as well as a full write-up of the R4 🙂