At present, there is no direct way to do this. But if you are reorder the portfolio using https://wordpress.org/plugins/post-types-order/ plugin, and put the items you want to show up on the top, you can use the post_count attribute of [show_portfolio] shortcode to display only the items you need –
[show_portfolio number_of_columns=4 post_count=12 image_size='medium' filterable=true no_margin=true]
Basically the post_count will limit the number of items seen and hence filter out the lower priority items as set using the plugin mentioned above.