Hi,
Yes, you can upgrade from v3 to v4 while keeping your change in CSS file but please note that there are some changes in class name which might affect the template layout as described here.
To update Font Awesome, you will have to update appropriate files in root_folder/templates/ja_argo/fonts/font-awesome first.
Next, backup folder root_folder/templates/ja_argo/css and navigate to Admin >>> Template Manager >>> choose your template >>> choose Compile LESS to CSS. After that, replace folder css with the original that you backed up before so only the change for Font Awesome will be kept.