Beware as the following has not been confirmed or denied yet by an official source and can be seen as SPOILERish rumors! So maybe we should take it with a grain of salt.

Ray Winstone

The LA Times published an article on Robert Zemeckis' upcoming CG film Beowulf in which Indy sidekick, British actor Ray Winstone, plays the title role. In the article it is briefly mentioned that Winstone's character in Indy 4 is named "Mac."

LA Times: Winstone is actually 50 years old, but thanks to wizardry of motion capture technology, he and other main characters age onscreen from teenagers to 70-year-olds as the film progresses. "It's been a long time since I've looked 17 or 18 years old," Winstone said, on a call from the set of Indiana Jones IV, in which he plays a character called "Mac."

It is unclear if LA Times got this name from Ray Winstone himself or from somewhere else.

Shia LaBeouf

Now about Shia LaBeouf's part; rumors are circling the web that the -- greaser; wearing jeans and a black leather jacket and riding around on a motorcycle -- character Shia LaBeouf appears to be playing is going by the named "Mutt". And if you look closely at that Comic-Con satellite video at IndianaJones.com, you can see such a name embroidered on Shia's black leather jacket. But it might not be "Mutt"! According to Mike Sampson from JoBlo.com, the name on the jacket is "Matt"; as from Matthew.

JoBlo.com: Other items you might want to keep your eyes out for: SPOILER [ Indy's son is apparently Matthew Jones ], given away by the name "Matt" embroidered on Shia LaBeouf's leather jacket.

The last name if Shia LaBeouf's character is a different rumor story for another time.

Thanks to Tim Wilson for the LA Times article link.