Hi Ryan,

Thanks for contacting us regarding your query.

Since this is a Visual Composer heading element, you will need to use Custom CSS Code to adjust the size on mobile.

You may use the following Custom CSS Code to accomplish this request. 

/* Mobile responsive support */  
@media (max-width:767px) { 
  h4.vc_custom_heading{
    font-size: 1.2em;
}    
}

You may add this to the Custom CSS Code section located by navigating to Appearance>General Settings>CSS/Script Related>Custom CSS Code from your WordPress Dashboard.

Please let me know if there are any more questions that I may answer for you.

Cheers! 

You need to use !important. 

Add this into the Custom CSS box located in your Salient Options panel (Make Sure there are no red cross in the Box) :

@media (max-width: 767px) {
    h4.vc_custom_heading {
        font-size: 1.2em !important;
    }
}

Thanks