The studio seems to be well aware of that fact in any case, judging by their attitude towards their superhero films so far. They certainly don’t seem to be taking their involvement in them lightly, and according to some fans and critics, the distasteful thing would have been to just spew out one film after another as quickly as possible for the sake of making some quick money.
It’s also interesting to note how the situation has evolved for them, and how external factors have affected their own work on their production. It will be interesting to see how they are going to handle “Iron Man 4” if Robert Downey Jr. really isn’t going to take part in the film.
While Marvel will most likely still find a way, it’s arguable whether the production will be the same in the eyes of many fans. Even if the plot is adjusted to accommodate the cast changes, many fans would still know what actually happened behind the scenes.
Let’s not make any drastic predictions like that though, as it’s entirely possible that “Iron Man 4” could still turn out to be a successful and enjoyable film. Anything is possible with a good dose of marketing, combined with hitting the screen at the right time. We’ll find out what the studio is planning to do in a few years anyway, so fans should be patient for the time being.