Well, I'm sure the official logo will have them on separate lines, separate fonts, or the like to differentiate them.
Personally, I think it's an interesting title. I'm curious to know what it means, and that's part of what makes a good title. If they'd just called it M:I4, all that would say is "this is the next unit churned out by the assembly line." It wouldn't say anything about it as a distinct story.
Although it's ironic that this is the film to break from the generic, assembly-line titling, because this is the only M:I film that really has any creative continuity with its predecessor, since Abrams is producing and wrote the story, even though it has a different director and screenwriters. It isn't really an assembly line when each product is so distinct from the others. The first film was a Brian DePalma paranoid thriller, the second was a John Woo action thriller, the third was Alias
on a feature budget. Which is exactly why their generic, numbered titles are so inadequate and unrevealing. This one will hopefully be distinctly a Brad Bird film, since Brad Bird films to date have been consistently excellent, but it will have an Abrams influence as well.