The limit of the 4 pound line depends on its physical makeup and the efficiency of the drag of your reel, and the action of your rod, and the skill of the fisherman and the conditions it is used in and the fish species you are after.
For example, if you've got a smooth quality reel and rod with some give and take, and are fishing in snag free water with a modern 4 pound test super line, then i'd say you could land a 20 pound plus fish (non-toothy of course) with 4 pound line.
But be warned, one hiccup with the drag, one submerged log or rock, one large toothy fish and 4 pound test line will snap with ease.
Another case scenario, i would never use 4 pound test to catch 6 pound large mouth bass. Because I know for a fact that the fish will dive into cover once hooked.