You could fail missions and, except for certain missions that were critical to the plot, your career was your own to manage. You could even opt out of the game's script entirely and be a true mercenary, working first for one house and then another. In MW4 you play a series of canned missions in one environment, then move to the next. For example, the first four missions are all on a small moon, where heat is almost irrelevant. The next four are in the arctic, the three after that in a temperate climate, then you move to the dessert, a very challenging place to run mechs.
Heat is as much an enemy as the opposing mechs.
There are 28 missions total in the campaign and you can't fail any of them. Nor can you save. The game automatically saves your progress at the end of each mission and you can go back and play from any of these saves at any time. On the plus side, Instant Action and Multiplayer have so many options that this baby won't be coming off my hard drive until somebody does it better. The FMV introduction is stunning.