During a recent project utilizing bootstrap 4, I encountered a similar challenge. One key aspect to consider is whether you are utilizing the img-fluid class within your card design. In order to ensure consistency in height among all cards, it's important to establish a minimum height value within the media query of your card class. For instance, setting min-height: 350px; can help maintain uniformity. Additionally, specifying a width for the card class within the 600px media query is crucial. To fine-tune this aspect, I recommend using Google Chrome's mobile responsiveness mode. By adjusting the width to 600px and experimenting with different pixel values, you can achieve the desired outcome. If you're unfamiliar with this tool, simply right click on your browser, select inspect, then switch to mobile view by clicking on the phone icon next to Elements. Within the mobile view settings, choose Responsive and set the max width to 600. Here is an example of the code:
@media only screen and (max-width: 600px) {
.card {
width: 48%;
min-height: 200px;
}