What is the best way to adjust the md, xl, etc usage mix in the col class of Bootstrap in order to achieve the following responsive folding behavior?
Wide screen:
+---+---+----+----+
| A | B | C | D |
+---+---+----+----+
| 1 | 2 | 3 | 4 |
+---+---+----+----+
| a | b | c | d |
+---+---+----+----+
Narrower screen:
+----+----+
| A | B |
+----+----+
| 1 | 2 |
+----+----+
| a | b |
+----+----+
| C | D |
+----+----+
| 3 | 4 |
+----+----+
| c | d |
+----+----+
Smallest mobile screen:
+-----+
| A |
+-----+
| 1 |
+-----+
| a |
+-----+
| B |
+-----+
| 2 |
+-----+
| b |
+-----+
| C |
+-----+
| 3 |
+-----+
| c |
+-----+
| D |
+-----+
| 4 |
+-----+
| d |
+-----+