Resident Evil is a remake of the original Playstation game. You play as STARS operatives Chris Redfield and Jill Valentine. Two police agents trapped inside a mansion crawling with zombies and mutant monsters. The game features beautiful graphics, but this is more than just a graphical update of an old game. This remake adds a ton of new content. The mansion the game takes place in has been rearranged. New areas have been added. Puzzles are also new. In some cases they're made precisely to fool veterans of the original game who think they have everything figured out. There's also new creatures, and new story elements. Controls may be a little hard to get into for newcomers. Resident Evil games have always been criticized because your character moves like a tank. You have to press a button to run, and because of the fixed camera angles, controls may get a little more frustrating for some. But the game definitely has more going for it than gainst it.
Resident Evil Zero is a prequel to the first game. Its events occur only a day before the events of the first game. In it, you play as STARS member Rebecca Chambers and escaped convict Billy Coen. And once again, you're trapped in an environment filled with undead nasties. Graphics are just as good as those in the REmake. Gameplay-wise, Zero is very much like the first game, but there are significant additions. One of them is the ability to switch control of either character on the fly. It's definitely a great game, but it's not as good as the REmake.
Resident Evil 4 skips two full chapters (or three if you count Code Veronica), and features a new storyline. That may be a bit disorienting for those whose previous RE experience only consists of the two previous games in this collection. The plot is indeed new, but the game does have references and allusions to what happened in episodes 1,2, and 3; and the game's lead character is Leon S. Kennedy, the main character from Resident Evil 2. RE4 is basically a reboot of the series. It features significant changes compared to previous games in the series. While movement controls remain the same, the camera angle is no longer fixed. It now follows your character from a behind-the-back or over-the-shoulder perspective. Its definitely an improvement over the control and camera issues the earlier games had. Enemies have also changed for the better. The iconic (but slow and dumb) zombies the series became famous for have been replaced by faster and smarter enemies. The graphics are stunning. It looks even better than the two previous games, and that's no small feat. RE4 is also much more action-packed than any previous installment, so much that the game has been criticized as being more of an action game than a horror one. I think the game is a perfect balance of both genres. While it is now more action-oriented and does feature lots of differences, at its core, it retains the tense feel of the original games. And it still does feature plenty of scares. There's nothing new here for those who already own these games in their original editions. There aren't additions of any kind. It's just a collection that brings in one package three of the best games in the series, and it's a perfect introduction for those new to this great survival horror series.