Where to kill 11+ enemies in a row to get 1UP (without glitch/turtle trick)?
The 4-1 example you gave looks like a place where this is possible. Use the green shell to box in the spinies, wait for Lakitu to generate 11+ spinies, then use the green shell to kill them all at once.
World 8-2 is a better place for this, since it generates green turtles naturally, rather than requiring a glitch to turn a spiny into a green turtle.