Which function allows Karel to move four steps at once?

The function that enables Karel to move four steps at once is indeed the one that is specifically named to reflect that action, which is move4(). This function is designed to take a single command that instructs Karel to move a total of four spaces in one execution. This is efficient as it streamlines Karel's movements, allowing for multiple steps to be completed with a single command rather than requiring four individual move commands.

Functions like move3() and move2() similarly cater to moving Karel a specified number of steps, but they are limited to three and two steps, respectively. Meanwhile, safeMove() typically indicates a movement command that checks for certain conditions (like avoiding walls or beepers) but does not imply adjusting the number of steps Karel can take in a single command. Therefore, move4() stands out as the correct choice because it directly fulfills the requirement of moving Karel precisely four steps in one function call.