Set/Override Padding In UICollectionView Between C

I have a UICollectionView and I'm running into an issue with getting the padding between cells. In theory, I should be able to divide the screen by 4, and I can get a cellsize that has 4 images which perfectly take up the screen width. But, it chooses not to do that. Instead it creates 3 images with HUGE padding as shown below:

_cellSize = CGSizeMake(UIScreen.mainScreen().bounds.width/4,UIScreen.mainScreen().bounds.width/4)

So I decreased the cell size a bit to create some padding, and this was closer but I need about half this padding:

_cellSize = CGSizeMake(UIScreen.mainScreen().bounds.width/4.4,UIScreen.mainScreen().bounds.width/4.4)

So I tried making the cells a bit bigger and it rolls over to the next row and puts 3 items with HUGE padding again.

_cellSize = CGSizeMake(UIScreen.mainScreen().bounds.width/4.3,UIScreen.mainScreen().bounds.width/4.3)

Can I specify the padding between cells without it deciding that if the padding is below "x" amount that it creates a new row instead? Like can I set the threshold padding before a new row to say, 0?

I want to make it this thin:

You only need to do the correct calculation that includes the spaces you want to the edges as well as the space between the cells. For instance, if you want 3 cells across with 5 points to the edges and between the cells, you should have something like this,

override func viewDidLoad() {
        super.viewDidLoad()
        var layout = self.collectionView.collectionViewLayout as UICollectionViewFlowLayout
        layout.sectionInset = UIEdgeInsetsMake(0, 5, 0, 5);
        layout.minimumInteritemSpacing = 5; // this number could be anything <=5. Need it here because the default is 10.
        layout.itemSize = CGSizeMake((self.collectionView.frame.size.width - 20)/3, 100) // 20 is 2*5 for the 2 edges plus 2*5 for the spaces between the cells
    }