Emma HayesThe reigning champions, who meet United at Wembley on Sunday, took control of the game thanks to Guro Reit’s eighth-minute finish and Erin Cuthbert’s 50th goal for the club.
Harder’s second brace in as many games – following a brace in Sunday’s 7-0 win over Everton – put the result before half-time in no doubt.
Lauren James extended the Blues’ lead with a stunning solo goal in the 56th minute before Jelena Cankovic completed the goal with another spectacular goal in added time.
Second-placed Chelsea, who have won four straight league games, have one game against title rivals United and have an identical goal difference.
Relegated Leicester, who were beaten 8-0 at home by the Blues in December, remain two points above bottom Reading with two games to go after losing 15 in a difficult season.
arsenal boosted their hopes of Champions League qualification by moving into third place after an early brace from Stina Blackstenius helped them win 4-0 Brighton.
Sweden striker Blackstenius struck in the sixth and eighth minutes to put the visitors on course for a comfortable win.
Frida Maanum and Victoria Pelova also scored first-half goals for the Gunners, who overtook Manchester City with a game in hand on goal difference.