The gameplay hasn't changed a bit either, and that's both a good thing and a bad thing. If you love old-school shooters, where absolute precision is key and the challenge never relents, then
Gradius III & IV may be the best bang for your purchasing buck. If, on the other hand, you grew up on the more recent style of shooters, where eye candy is key and insane reflexes play second fiddle, you'll have a hell of a time getting anywhere in these two games.
The core gameplay in both titles is quite similar. You control the Vic Viper, a state-of-the-art spaceship, on a quest to... blow stuff up. There's no plot, but that's not the point. There's a bar at the bottom of the screen that shows the power-ups available for the Vic and your current location on the bar. Every time you pick up a red power-up icon, you move up one space on the bar. Pressing the Square button on the controller uses up all of your power-up icons and gives you whatever ability is highlighted. Every configuration has Speedup, and there's a general pattern that the rest follow: missile, dual shot, laser, Option, shield (and GIII has a "special" after that). Options are the only one that really needs explaining -- they're basically ghost ships, which follow your actual ship around and fire the same weapons you have when you fire them. Their main benefit is that they can't get destroyed. Of course, your ship can generally only take one hit, so you've got to be very, very careful when you play.
The level structure and power-up structure differ between the two games. Gradius III offers a "build-your-own" power-up setup, whereas GIV forces you to pick from a set of pre-builts. Luckily, my favorite setup is a prebuilt already, so I have no problem with that. (In case you're curious, it's torpedo spread ripple option shield. Say that ten times fast.) And the first level of GIII is sand, whereas in GIV it's floating golden balls of dragon doom. Bubbles appear in Gradius III's second level, whereas they wait until Gradius IV's third to show up. But, despite the differences in the bosses and level structures, the basic gameplay experience is quite similar.
Despite the old-school gameplay, there's a ton of fun to be had with these two games. I played the hell out of the SNES version of Gradius III, and I'm glad to see it on a current console. And I'd only played Gradius IV for about two minutes before this. The games keep track of how much time you've logged in them, and offer to save your high-score data and other stuff on your PS2 memory card. At 78K, it's one of the smallest saves you'll ever see.
There are also a few special things that you can really only get on Gradius III & IV. You can set the DIP switches to however you like, making the game "easy" or "hard" (read: impossible or really, really, really impossible), jacking up the number of lives you start with, and even slowing down GIII some. It also sports a Stage Select mode where you can start at any stage you've already gotten to. GIV also sports a Boss Attack mode that lets you go after all of the bosses without all that intervening "level" stuff. But those features are just icing on the cake.