It would help us a lot to help you if we knew a little about what you're trying to do, what hardware you're using, and, importantly, where you are?
If you had a PAL disc and have bought an NTSC one, then the chances are the disk won't even load GT4 as the formats are not cross-compatible, let alone the gamesaves.
If you remove GT4 disk from your system and start it up, go to "Browser" and look for a GT4 savegame on your memory card. You should be able to select it and it'll tell you when it was last saved and how big it is. It should be ~1500kB.
If that's not present, then you have no savegame on your mem-card and that's why the game can't find it.
Can you tell us where you are? What model of PS2 you're using (slimline or fatty?) and what your GT4 disc says on it (or what it says on the GT4 box directly under the multicoloured PlayStation Logo?) Is it PAL, NTSC U/C, NTSC J, etc?
Also could you take a screenshot of the error it's giving and post it here so we know exactly what you're talking about?
If your gamesave is indeed corrupted, it's unrecoverable, and there's really nothing we can do to help, except offer commiserations, and tell you to start a new gamesave, and, importantly go into "Options" and turn OFF the 🤬 autosave! (Which is usually the culprit in corrupt data issues as it tends to save stuff unexpectedly leading to people powering down their consoles while it's saving and corrupting their data!)
If you're in NTSC land, there's a couple of threads on this board, one by me and another by GrumpEone that have completed gamesaves in them for your to download, however, you'd need a GameShark / SharkPort or a MAXDrive and appropriate software to get them back onto your PS2 Memory card. I think there's some completed PAL saves out there too, but I'm not sure where...