How it works...

The flex-start value of the justify property aligns all of the Grid items at the start of the container. In this case, the three Chip components in each of the four containers are all crammed to the left of the row. None of the space to the left of the items is filled. Instead of changing the breakpoint property values of these items, which results in changed widths, you can change the justify property value to tell the Grid container how to fill empty spaces.

For example, you could use the center value to align Grid items in the center of the container as follows:

<div className={classes.root}>
  <Grid container spacing={4}>
    <Grid item xs={12} sm={6} md={3}>
      <Paper className={classes.paper}>
        <Grid container justify="center">
          <Grid item>
            <Chip label="xs=12" />
          </Grid>
          <Grid item>
            <Chip label="sm=6" />
          </Grid>
          <Grid item>
            <Chip label="md=3" />
          </Grid>
        </Grid>
      </Paper>
    </Grid>
    <Grid item xs={12} sm={6} md={3}>
      <Paper className={classes.paper}>
        <Grid container justify="center">
          <Grid item>
            <Chip label="xs=12" />
          </Grid>
          <Grid item>
            <Chip label="sm=6" />
          </Grid>
          <Grid item>
            <Chip label="md=3" />
          </Grid>
        </Grid>
      </Paper>
    </Grid>
    <Grid item xs={12} sm={6} md={3}>
      <Paper className={classes.paper}>
        <Grid container justify="center">
          <Grid item>
            <Chip label="xs=12" />
          </Grid>
          <Grid item>
            <Chip label="sm=6" />
          </Grid>
          <Grid item>
            <Chip label="md=3" />
          </Grid>
        </Grid>
      </Paper>
    </Grid>
    <Grid item xs={12} sm={6} md={3}>
      <Paper className={classes.paper}>
        <Grid container justify="center">
          <Grid item>
            <Chip label="xs=12" />
          </Grid>
          <Grid item>
            <Chip label="sm=6" />
          </Grid>
          <Grid item>
            <Chip label="md=3" />
          </Grid>
        </Grid>
      </Paper>
    </Grid>
  </Grid>
</div>

The following screenshot shows what this change to the justify property value results in:

This does a good job of evenly distributing the empty space to the left and right of the Grid items. But the items still feel crowded because there's no space in between them. Here's what it looks like if you use the space-around value of the justify property:

This value does the best job of filling all the available space in the Grid container, without having to change the width of the Grid items.