Even before Knight And Day made its bow at the US box office a few weeks ago, there was a sense that vultures were circling Paramount's soon-to-shoot blockbuster Mission: Impossible 4. After the very slightly disappointing gross of Mission: Impossible: III, Paramount was believed to have considered making the fourth movie without Tom Cruise as its leads star. That's something the franchise could quite easily take, you'd wager, given that it's probably less movie star-dependent than it might appear.

However, then word came through that Tom Cruise was, indeed, set to reprise his role as Ethan Hunt in the fourth movie, and after scouring a promising list of directors, Paramount then plumped for what could be the quite inspired choice of Brad Bird (The Incredibles, The Iron Giant).

Yet then, a problem arose. Because this summer seems to have proven that Tom Cruise simply isn't the box office draw that he once was. In fact, you could further argue that he's not had a massive box office hit that's met its full potential since 2005's War Of The Worlds. Certainly Valkyrie suffered from long delays before arriving to decent, if unspectacular box office. And his supporting role in Lions For Lambs pretty much sank without trace.

What's probably caused more than a few meetings at Paramount, however, is Knight And Day's performance. The film, in spite of costing around $120m for the negative, has currently taken less than $70m in the US, and it's likely to have dropped out of the top ten next weekend after a month on release.

Granted, the film still has its overseas roll out to reprieve it (as happened with Prince Of Persia: The Sands Of Time, a modest release in the US, but a big success elsewhere in the world), but nonetheless, given that Cruise isn't the cheapest leading man on the market, Paramount is seemingly asking questions (although, crucially, he has produced all of the Mission: Impossible films too).

As such, the strong suggestion is that the script for Mission: Impossible 4 is now being rewritten to bring in a supporting character for Ethan Hunt, one that he can groom, presumably to take the lead in the franchise for a fifth movie. Expect Paramount to hunt for a rising star for that role, and given that cameras are expected to role in September, expect the identity of the actor or actress to take it to be revealed shortly.

Meanwhile, The Hollywood Reporter has given us a further piece of casting news. For it's revealed that Simon Pegg is set to reprise his role as Benji Dunn from Mission: Impossible III. JJ Abrams is on producing duties (having directed the last film), and more information on the film is expected shortly.

We will, of course, be keeping you posted. In the meantime, here's that story on Mr Pegg's involvement.

Mission Impossible 4 is set for release on 16th December 2011.