Final answer:
To determine when a rocket will be at y = 4 meters during launch, one must use the kinematic equation for displacement with the provided acceleration, assuming an initial velocity of zero if not specified differently.
Step-by-step explanation:
The question 'when will the rocket be at y = 4' belongs to the field of Mathematics, specifically in the area of kinematics, which is a part of physics but often studied as a mathematical concept in high school. This problem requires an understanding of the equations of motion. The information given suggests the need for an equation that links the displacement (y) to time (t), acceleration (a), and initial velocity (usually denoted as u, which might be 0 in this case since it's during launch).
Usually, the formula y = ut + ½ at² is used, where u is the initial velocity, a is the acceleration, and t is time. Using the given acceleration and assuming an initial velocity of 0 (since the question doesn't specify otherwise), we can set the displacement y equal to 4 and solve for t. As the acceleration a is 20 m/s², the equation would then be 4 = 0*t + ½ * 20 * t². Solving this equation for t would give us the time at which the rocket reaches the height of 4 meters.