To change font size of h4 on specific text block, you can follow these.
First, find Text Block ID. In my example, it is: #block-yui_3_17_2_1_1739950653718_1742
Next, use this code to Custom CSS box
#block-yui_3_17_2_1_1739950653718_1742 h4 {
font-size: 50px !important;
}
Result
