Sonic 4 Episode I and II are downloadable games released on numerous platforms including PC and iphone. Here are my thoughts.
In my opinion Sonic 4 Episode I is more like Sonic Remix. It features a handfull of levels which can be described as remixes of previous levels in previous games. It even includes the same enemies and even the same bosses! Graphics are okay but opening Splash Hill backgrounds feel dull washed out. In Splash Hill for example, the background are paper cut outs! It's probably not noticeable on an iphone but for on a PC or console it looks horrible!
It feels like its struggling because it was developed for handhelds and the iphone which have less processing power rather than just consoles. Instead of using 3D models which don't look that great I personally wouldn't have minded if they had used pixels and sprites instead.
Episode II feels more like its own game. Levels feel better and less recycled and better paced. The inclusion of Tails with co-op and the ability to use Sonic and Tails moves was a good move. Graphics have been improved greatly and now feel brighter, shinier and include more animations. Bossess are completely new and not copied from other Sonic games.
Metal Sonic levels are unlocked if you have Episode I but these are just levels from the first game which are a bit harder and aren't really that exciting.
As a package, overall, Episode 1 is skippable in my opinion. You're better off just playing Sonic 1,2 or 3 & Knuckles. Episode 2 on the other hand feels like a solid attempt at a Sonic game. The Metal Sonic levels don't really add much to the game so I would say save your money and just get Episode II if you must get Sonic 4. You also only get 4 zones per episode which makes each Episode feel like half of a Sonic game. Don't buy these games, if you're playing on the PC and console, you're better off just getting Sonic Generations, the far superior game which has both classic Sonic and modern Sonic and which was designed to be a console/PC game.