A floated sidebar, contains a table. No problems there, as the sidebar container has a (pixel-) defined width. A main contents area, with a red border (static <div>, width defined only by margins and padding).
The tables should be as wide as the main contents, and not drop under the sidebar. Except for the first table (control table), all tables are wrapped in a wrapper div (width:99% declared). Just to make sure that the browser's lack of support for border-collapse:collapse and border-spacing:0 doesn't affect the results: the following tables have the html properties cellspacing, cellpadding and border hard coded in the page, and set to 0 (zero).
a table, no width is declared; without a wrapper
| cell1 | cell2 | cell3 | cell4 |
| Lorem ipsum dolor sit amet, consectetuer adipiscing elit. | Aliquam ultricies, metus sed rutrum dignissim, leo dui sollicitudin sapien, | sit amet sagittis est metus ut erat. | Maecenas sollicitudin. Lorem ipsum dolor sit amet, consectetuer adipiscing elit. |
a table, width:100% is declared, within a wrapper (width:99%)
| cell1 | cell2 | cell3 | cell4 |
| Lorem ipsum dolor sit amet, consectetuer adipiscing elit. | Aliquam ultricies, metus sed rutrum dignissim, leo dui sollicitudin sapien, | sit amet sagittis est metus ut erat. | Maecenas sollicitudin. Lorem ipsum dolor sit amet, consectetuer adipiscing elit. |
a table, width:100% is declared and equally size cells (td width:25%), within a wrapper (width:99%)
| cell1 | cell2 | cell3 | cell4 |
| Lorem ipsum dolor sit amet, consectetuer adipiscing elit. | Aliquam ultricies, metus sed rutrum dignissim, leo dui sollicitudin sapien, | sit amet sagittis est metus ut erat. | Maecenas sollicitudin. Lorem ipsum dolor sit amet, consectetuer adipiscing elit. |