To change font size of h4 on One Page. You can follow these.
#1 . First, find Page ID.
In my example, it is: #collection-67b563bb6d64095fa49aa748
#2 . Next, use this code to Custom CSS box.
#collection-67b563bb6d64095fa49aa748 h4 {
font-size: 50px !important;
}
#3 . Result
