valid_styles
seems well documented for version 4 :
This option enables you specify the available styles for each element. This means you can force ordering and only specific styles to be valid within style attribute values. Specify the element name, then it's styles or * for all elements.
tinymce.init({
...
valid_styles: {
"*": "border,font-size",
"div": "width,height"
}
});
Here is a tinymce fiddle you can play with.