Shortcode should be like, [aec_events view=grid limit=4 category=ID]
Kindly refer
yendif.com/wordpress/another-events-calendar/shortcodes.html for more shortcode attributes.
Where can I find the ID of the category
I understood it's hard to find the Category ID. We use WordPress's built-in taxonomy option for categories and the built-in taxonomy GUI itself doesn't show the ID's. Kindly refer
bobwp.com/find-post-page-category-id-wordpress/ to get the category ID. Note that this link explains the steps to find the WordPress's native Category ID. You will need to follow the same steps but under our plugin's Categories menu.