Taking advantage of unused sidebar space?

Is it at all possible to make my posts on the main page fill the full width of the page after the content of my sidebar ends? Right now, after about the 2nd post on my home page, there’s just this empty space to the right (filled above by my content in the “sidebar top” area); it’s aesthetically displeasing and also a poor use of space. Not sure if this is related, but I haven’t really figured out the “grid” thing either – I chose to display 4 posts before the grid begins, but when I scroll down my posts (after the 4th) just display consecutively as usual.

In case this may be causing the problem: note that, as suggested elsewhere in these forums, I’ve modified the CSS for the standard posting option to display the content (as opposed to the summary version) so that I can choose where the “read more” link appears.

Thanks in advance for your help!