Sonic 4 was released in 2 parts, with years between the episodes. The series is a return to the 2D gameplay that fans craved. The graphics and animations have improved a lot since the first episode, although I encountered a graphics problem where there were green graphical artifacts. The physics are much improved too, and it feels more like a standard Sonic game.
This game has new ideas for the levels and bosses so seems a completely different game to the first episode. There are some nice themes like an oil platform, or a castle, and the bosses usually have a large scale to them.
I think the levels flowed quite nicely. A lot of Sonic games seem like they encourage speed but then add many annoying aspects to slow you down. This time, Tails joins Sonic on his journey and he often behaves like he did in the old games where he constantly runs into danger or falls down pits. He respawns quickly though. You have a few moves you can do together. The first is where Tails picks you up to temporarily fly. The second is turn into a ball for a fast dash and powerful attack. When these moves are required, there are TV screens prompting you to do the maneuver. Some players will be annoyed at this hand-holding. There are also signs to point out where there are bottomless pits. I actually liked this feature as many Sonic games see me jumping down gaps only to find out it leads to my death.
The menu has a level select so you don't even need to play the acts in order.
I still don't like the lives system. If you aren't good at the game, you could run out of lives and then have to restart the level from the beginning, but now with a small stock of 3 lives; which then makes it more difficult for you. I found the majority of levels to be really easy, but there were a few sections in the game where it wasn't obvious what you needed to do, so I lost several lives in these sections.
Sonic games often have good music, but these songs just sounded like the same several seconds stuck on loop. It grows old really fast.
This game is a massive improvement over the previous game, with more ideas and fewer annoying aspects. However, it still isn't that interesting compared to other platforming games. There's too many moments where you just hold right, and occasionally press jump when required.