As far as I know the only way to save a GP4-replay is to save the complete savegame on your harddrive.
Be careful:
- When you re-load the game, GP4 cuts the replay-buffer at 30 minutes and sometimes it has a bug so that there will be only about 20 seconds available in the replay. So the more often you save the game, the better it is.
- When you load a savegame you have to load it to 100% on the same track as the savegame is originally from. Even a difference in the magic file will screw it up completely. Also the general settings like the dirty air effect have to be the same.