A mkr is hugely better than a Mk3.

Main benefits:
3D-perspective view
Heaps faster
Better graphics
Can read latest maps on dual-layer DVDs
Customisable - you can add custom POIs easily

V32 is the latest version of the operating system software. V32 contains 4-1/100 software for the Mk4.
Whether you need it or not depends what your Mk4 comes loaded with. If it has 4-1/100 fitted, you don't need V32 OS disc. If older than 4-1/100, then updating the software is beneficial (new features, bug fixes, etc)