After slowly catching city rivals Roma in recent weeks, Sunday's comprehensive win saw them finally move above Rudi Garcia's men - held 1-1 at Torino - by one point.
Goals from Stefano Mauri, Miroslav Klose and Antonio Candreva put Lazio firmly in control before the break at the Stadio Olimpico, with the prolific Felipe Anderson adding a fourth before Diego Novaretti was shown a second yellow card.
Lazio may have finished with 10 men, but their victory was never in doubt, while the same could not be said for Roma, who saw Torino's Maxi Lopez cancel out Andrea Florenzi's penalty.
That point sees Roma relinquish second place for the first time this season, while Napoli boosted their hopes of qualifying for Europe with a 3-0 win over rivals Fiorentina.
Their encounter was not without controversy, though, as Gonzalo Higuain saw a goal not given by the officials – who do not have goalline technology to help them – despite the ball landing behind the line after hitting the bar.
Luckily for Rafael Benitez it did not matter as Dries Mertens, Marek Hamsik and Jose Callejon moved Napoli above their opponents after their first win since late February.
Atalanta boosted their survival hopes with a crucial 2-1 win over Sassuolo thanks to a German Denis double as both sides ended with 10 men, while Chievo piled more misery on Cesena.
Sergio Pellissier struck the decisive goal in Chievo's 1-0 win, leaving Cesena seven points from safety with eight games remaining.
Meanwhile, Udinese's poor run of form continued as they were beaten 3-1 by Palermo, a fifth game without a win for Andrea Stramaccioni's side.
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