It's on the right track, but not quite there yet
First of all, Sonic's physics are all over the place. He's either too heavy or too light. I mean, it takes a while for Sonic to actually reach top speed on this one. Even after hitting a booster, Sonic still doesn't feel as fast as his older games, although I've got to admit I did like the platforming elements of swinging from vine to vine, although its not clear the first time that you have to jump off when it stops moving for better results. Sonic's homing attack is also new to 2D Sonic, in where the player presses A while in the air to homing attack an item or an enemy. Most of the time it works, but occassionally it doesn't, and sometimes if it doesn't work, it usually means you end up falling to your death.
All of Sonic 4: Episode 1's levels are based on older Sonic games, which could be considered a blessing or a curse to gamers. Sonic's first stage is basically a cross between Emerald Hill Zone and Green Hill Zone, which isn't a bad thing because the level's transition is quite good in HD. The Casino level is almost exactly the same as Sonic 2's version, but it adds some Carnival Night Zone elements into the level too. The Lost Labyrinth Zone is a hybrid of Labyrinth Zone and Sandopolis Zone, and Mad Gear Zone is another hybrid of Chemical Plant Zone and Metropolis Zone. I'm not bashing these levels, because it's a good start, but should this game really be called Sonic 4 some may argue?
Difficulty wise, the game is a little all over the place. The first two stages are fairly easy, though when you get to Lost Labyrinth, the difficulty shoots up like a rocket almost. Filled with cheap deaths and lots of pits and spikes, Sonic 4 gets quite difficult, though to be fair, the game does have alot of easy ways to obtain lives, though I will say Mad Gear Zone's Robotnik boss is a bit annoying, since you have to be in the right place just after you kill him to avoid getting killed, and if you get it wrong, you're dead.
The special stages are back, and they follow the Sonic 1 style of navigating through a maze to the emerald. You get to rotate the maze with the analog stick to choose which direction the gravity falls Sonic into. If he hits the Exclamation Mark GOAL squares, he loses and exits the stage. Sonic also has a time limit this time so players must be quick to get the emerald. Later levels become incredibly tough in completing, since you usually end up falling into a GOAL square off the screen, and to complete it usually requires Trial And Error to finish.
Graphics and Sound wise, Sonic 4 is good but not the best. Some of the music is good, though others are very annoying and repetitive (Two of the music tracks from Lost Labyrinth Zone). Graphics are in full HD and Sonic uses his skinny half-3D character sprite for the game, though the backgrounds are quite beautiful in places (Casino Zone). Overall, the game looks good, but I do like the 16 bit ones better in my opinion.
Overall, I did enjoy Sonic 4: Episode 1. It takes about 1 to 2 hours to finish normally, though getting the emeralds is fun and being Super Sonic rocks as usual. At the price of £10, it might seem a little steep to some gamers, but I think there's actually a fair amount of replay value on this game. Hopefully, we can assume Sega will make the game even better with future episodes, but overall, the game is about an 8/10 at best.