Problem with Shop items alignment

Hi. I'm having problems with the alignment of my product thumbnails on my shop page. I have the shop page set to display four columns, but it only display three columns. The second row only has one shop item on it. Then the third row has three. Is there a way to fix this to that each row has the maximum number of items? The second row will only display one item no matter how many different configurations I try to use.

Thanks for your help,

Krystal