This setting can be found on the the file jcarousel.responsive.js, here is the Responsive Carousel example on GitHub https://github.com/jsor/jcarousel/tree/master/examples/responsive.
The lines
if (width >= 600) {
width = width / 3;
} else if (width >= 350) {
width = width / 2;
}
determine the quantity of items that will be shown on screens larger than 600 and 350 respectively, to show 4 elements on larger screens and only 3 on smaller screens (mobiles) you can use:
if (width >= 600) {
width = width / 4;
} else if (width >= 350) {
width = width / 3;
}
Also note that some users seem to change the setting "visible" (Setting number of visible images in jCarousel) this variable seems to make no changes on the responsive example.