  In Real Analysis, Littlewood's three principles are known as heuristics that
help teach the essentials of measure theory and reveal the analogies between
the concepts of topological space and continuos function on one side and those
of measurable space and measurable function on the other one. They are based on
important and rigorous statements, such as Lusin's and Egoroff-Severini's
theorems, and have ingenious and elegant proofs. We shall comment on those
theorems and show how their proofs can possibly be made simpler by introducing
a \textit{fourth principle}. These alternative proofs make even more manifest
those analogies and show that Egoroff-Severini's theorem can be considered the
natural generalization of the classical Dini's monotone convergence theorem.
