All new animation
With Mecanim, our new animation system, you can animate any character or object with astonishing ease. Animate even vast armies with retargeting and Unity’s stability and power, combined with new optimizations, such as skinned mesh instancing, ensure smooth runtime performance. Easily construct and edit complex state machines and blend trees for complete control of how your characters move.

For true professional polish, use Pro-only IK rigs to fine-tune each detail in your animations: perfectly position legs throughout a jump sequence, or make hands grip a ledge, wrap around a doorknob or pick up objects exactly how you want them to. More about Mecanim

Gorgeous graphics
DirectX® 11 rendering
More beauty and great performance for your games. Unity 4 includes support for Windows DirectX 11 graphics API, to bring higher visual fidelity and faster rendering times to your professional PC games.
More about rendering

Updated Image Effects
Depth Of Field and Motion blur are optimized for DirectX 11, and key improvements made for Edge Detection, Bloom, Vignetting, Tonemapping, and Color correction using 3D texture lookups.
More about special effects

More for mobile games
Add more realism to your mobile games by casting Pro-only hard dynamic shadows from directional lights. And use dynamic fonts to efficiently render any character set at any resolution.
More about shadows

A fine-tuned Shuriken Particle System
Unity’s curve and gradient-driven modular particle system tool has been ramped up so you can create awesome explosions, collisions, weather storms and more on a tight performance budget. World collision functionality, Bent normals, automatic culling and external forces keep performance smooth while increasing the visual fidelity and dynamism of your particle systems. More about Shuriken

More detail and control with your lightmapped scenes
Unity’s iterative lightmap baking gives you complete control of your lightmapping workflow. And now, you can easily add more detail and nuance to your lightmapped scenes with Pro-only lightmapping of normalmaps, and keep performance smooth with lower memory requirements for baking. More about lightmapping

Performance
Refined profiling for mobile
Unity 4’s Pro-only Profiler now reports on GPU usage for games running on Android devices with Tegra chipsets. Turn on the Profiler while in play mode, and quickly pinpoint those performance-heavy areas in your game that will have the biggest impact on performance.
More about performance

Multiplatform
New publishing platforms
Bring your game to two new global platforms. The new Unity add-on for Adobe Flash Player ports your game to a platform installed on over a billion PCs. Or create an awesome desktop game for the growing legions of Linux players, who are hungry and ready to pay for great content. More about multiplatform

And many more new features including:
Render efficiently any character set at any resolution with cross-platform dynamic fonts
Configure a default cursor and change cursors on demand with new Hardware cursor API
Quickly add components to objects with Add Component button
Fast asset search and management with a new project browser
Script access to the output from a skinned mesh renderer to cheaply render multiple instances of a mesh in the same pose