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It's not clear what BioWare has planned for Mass Effect 4 and beyond. Regardless of what direction they take the series, though, they suggest that you hang onto your Mass Effect 3 save files.
"It wouldn't be a bad idea [to keep your ME3 saves]," associated producer Mike Gamble told GamerZines. ""Obviously I can't say anything, but it wouldn't be a bad idea..."
Mass Effect 2 used data from ME1 saves to alter the story. For example, if you rescued a character in the first game, he might come to you for help once again in the sequel. Mass Effect 3 will continue this trend.
Will Mass Effect 3 save files affect Mass Effect 4? That's harder to say. Assuming that ME4 is a couple years away, it might be released on a new generation of consoles. That would make transferring the data from ME3 much more difficult.
Mass Effect 3 save files would likely alter ME4 in different ways than, say, ME1 saves altered ME2. ME3 is supposed to be the last one starring Commander Shepard. BioWare is indicating that the fourth game could be a prequel, which would make the player's actions in the original trilogy irrelevant.
Regardless of whether your ME3 data ultimately comes in handy, you may as well keep them, though. It's not like it'd cost you anything.