Editing Magento grid view items per row isn't just like changing td-s in a table. The items are laid out as li-s and not a table. The parameter is taken from somewhere, but where, I haven't been able to find. 

So, I needed to have 3 items per row in grid view instead of 4. Here's how to change in grid view the number of products per row (tested in 1.6.1.0):

1. Open \app\design\frontend\default\your_theme\template\catalog\product\list.phtml
2. Find (near line 89) "$_columnCount = $this->getColumnCount();"
   
3. Replace it with "$_columnCount = 3 ; //$this->getColumnCount();" like:
4. The number value of $_columnCount represents the number of items per row in product grid view that you want to have. Replace it with what number you want.
5. Save and refresh page.