First-period goals from Abbie Russell and Sorcha Clarke set UCD on their way to victory at Queen's.
Deidre Duke's fine reverse stick finish made it 3-0 before Katie Mullan added the fourth.
Hermes-Monkstown are a point behind UCD after beating third-placed Cork Harlequins 4-3 while Ulster Elks lost 1-0 to Railway Union to remain bottom.
Monkstown have a game in hand on UCD with the two sides still to play each other as well.
The league's top scorer Anna O'Flanagan helped herself to two more goals with Nikki Evans and Ellen Curran also finding the net for Monkstown.
Railway Union moved above Pegasus and into fifth place after Niamh Carey scored the only goal of the game at Jordanstown as the Dublin side beat Ulster Elks 1-0.
The Elks remain rooted to the bottom of the table and a point behind Pembroke Wanderers, who lost 3-0 to Loreto.
The team that finishes last will be relegated to their provincial league next season while the side that finishes ninth must face a promotion/relegation play-off.
The result also sees Loreto strengthen their grip on fourth place and the final place in the play-offs.
In Friday night's game a Chloe Brown goal from a penalty corner in the final minute was enough to give Ards a 1-0 win over Belfast Harlequins that keeps alive their outside chances of making the top four.
There was just the one rearranged game in the men's IHL as Monkstown beat Railway Union 3-2, which leaves a five-point gap between Railway and Instonians at the bottom of the table, but the Belfast side have two games in hand.
Pegasus' hopes of reaching the Irish Hockey League play-offs were dented after they lost 4-0 to leaders UCD.