Testimonial Grid Section with HTML, CSS Grid

Looks good! Very clean!

A few things that can help with overall layout and centering:

body {
  display: flex;
  flex-direction: column;
  align-items: center; /* center content in column horizontal */
  justify-content: center; /* center content in column vertically */
  margin: auto;
}

For the responsive bit, don't use fixed widths (you're grid is repeat(cnt, size). Use something like 1fr in place of your size instead, which will tell the browser to evenly divide each column into 1/4th of the available space). And then maybe add a max-width to your wrapper (main in your case). You can also likely reduce your rows to 2, unless you have a specific reason for wanting that many.

Then in your media queries, you'll want to redefine (or remove, depending) the grid/row setting for each card.

Hope that helps!