Set Base URL to background image path in External CSS

Any one can help me to add base url to my background image path in my external css. My code like below. 

PHP Code:

aside.main-sidebar{
    background-image: linear-gradient(90deg, rgba(0,0,0,0.4), rgba(0, 0, 0, 0.2)),
            url(/ui/assets/img/bg4.jpg) !important;
    background-size: cover;
    background-position: center;
}