The story is ["Four in One"](https://www.isfdb.org/cgi-bin/title.cgi?46235) by [Damon Knight](http://en.wikipedia.org/wiki/Damon_Knight). [This blog post](http://variety-sf.blogspot.fr/2009/10/damon-knight-four-in-one-novelette-how.html) has a summary that matches your description.