All boxes have float: left, a fixed width (but a variable height) and a margin

Mozilla, Opera, Safari render the boxes like this:

--------------    --------------
|   Box 1    |    |   Box 2    |
|            |    |            |
|            |    |            |
|            |    --------------
--------------

--------------    --------------
|   Box 3    |    |   Box 4    |
|            |    |            |
| clear:left |    |            |
--------------    |            |
                  --------------


IE7 renders it like this:

--------------    --------------
|   Box 1    |    |   Box 2    |
|            |    |            |
|            |    |            |
|            |    --------------
--------------
                  --------------
--------------    |   Box 4    |
|   Box 3    |    |            |
|            |    |            |
| clear:left |    |            |
--------------    --------------