Wordpress - Portfolio - Change number of items in a row

I am using the Pure theme. Instead of having five (5) items per row in the portfolio, I want to have four (4).

How could I customize the theme's CSS? To build the page, I am using the portfolio module of the GT3 Page Builder.The CSS is below:

/* Portfolio */

.optionset {
    margin: 0;
    padding-left: 0;
    text-align: center;
    line-height: 13px;
}

.optionset li {
    list-style: none;
    margin-left: 0 !important;
    margin-right: 30px;
    display: inline-block;
    line-height: 13px;
}

.optionset li:last-child {
    margin-right: 0;
}

.optionset li:before {
    display: none !important;
}

.optionset li a {
    color: #484848;
    font-size: 13px;
    line-height: 13px;
}

.optionset li a:hover, .optionset li.selected a {
    color: #bfbfbf;
}

.portfolio_item {
    width: 20%;
    margin-bottom: 40px;
}

.portfolio_item .innerpadding {
    padding-left: 50px;
}

.portwrap {
    margin-left: -50px;
}

.portfolio_block {
    margin-top: 68px;
}

.isotope_preloader {
    /*opacity: 0;*/
    min-height: 700px;
}