Consult your theme description here http://theme.wordpress.com/themes/forever/
Featuring posts in a slider means creating several sticky posts http://en.support.wordpress.com/posts/post-visibility/#sticky-posts with featured Images http://en.support.wordpress.com/featured-images/#setting-a-featured-image of the correct dimensions required for the theme. Every sticky post you have (up to 10) that has a Featured Image (at least 887 pixels wide and 355 pixels in height) attached will show up there.
To show only your four latest posts (with a Featured Image above each one, of course) in a neat grid on your home page just visit Appearance → Theme Options and click the checkbox there. Thumbnail on 4-column showcase for recent posts. Width 195px, maximum height 124px (cropped if larger).
Featured Image (at least 887 pixels wide and height 355px)
The four latest posts are you four most recently published posts so they will change when you publish new posts.