The likelihood of staff changing that to "unlimited" or some high number is remote since it could introduce all sorts of issues on successive pages, and this is something that is set in the underlying theme PHP script files and we do not have access to those.
And this is just my thought, but past 4 featured posts no longer qualifies as "featured" but becomes the standard design for post display.
You could achieve something similar in appearance if you had CSS experience and then bought the Custom Design upgrade.
http://en.support.wordpress.com/custom-design/
http://en.support.wordpress.com/custom-css/
http://en.support.wordpress.com/editing-css/