I wanted to follow up on this because I found something that may be of benefit to someone else:
First off, once I had selected Bootstrap 4, an option for "control group class" appeared that I had not seen before.
In that field, I put "fa fa-user"
I've given my form a specific class ".BasicContactForm"
In my custom CSS file, I add the following:
Code:
form.visform.BasicContactForm [class*='fa-']::before {position: absolute;line-height: 30px;margin-left: 10px;font-size: 20px;top:5px;color: rgba(0, 0, 0, 0.4);z-index: 999;}
form.visform.BasicContactForm [class*='fa-'] .form-group input, form.visform.BasicContactForm [class*='fa-'] .form-group [class*='selection__rendered'], form.visform.BasicContactForm [class*='fa-'] .form-group textarea {font-family: 'Montserrat', sans-serif!important;font-weight:normal!important;padding-left: 40px;}
form.visform.BasicContactForm [class*='-container'] span span[class*='-selection'] {height:38px!important;line-height: 38px;}
This may not work for everyone, but it just might get you in the ballpark.