Bottom club Troyes were relegated to Ligue 2 after being beaten 1-0 at home by Angers.
Ibrahimovic scored twice for PSG in the first half and once in the second while David Luiz was also on target. Nice replied through Hatem Ben Arfa in the 18th minute.
PSG, who were crowned champions last month, have 80 points and with six games left lead second-placed Monaco, who lost 2-1 at home to Girondins de Bordeaux on Friday, by 25 points.
Nice, who occupy fourth spot, are one point behind third-placed Stade Rennais who defeated Stade de Reims 3-1.
Both teams, however, will be leap-frogged by Olympique Lyonnais (49 points) if they win at Lorient on Sunday.
There was a minute's applause at the Parc des Princes when former Netherlands great Johan Cruyff, who died last week, appeared on the giant screen.
Having lost and drawn their last two league games, anything but a win would have damaged PSG's confidence before they take on City in their quarter-final first leg on Wednesday.
Ibrahimovic opened the scoring in the 15th minute with a powerful cross-shot after being set up by Luiz.
Ben Arfa, who hopes to make France coach Didier Deschamps' 23-man squad for Euro 2016, replied with a superb curling shot that beat Kevin Trapp.
Sweden striker Ibrahimovic put Laurent Blanc's side ahead again by driving an 18-metre free kick into the top corner in the 34th minute.
Luiz then gave PSG a two-goal lead three minutes into the second half with a downward header from a corner.
Ibrahimovic made it 4-1 with eight minutes to go as he latched on to Adrien Rabiot's cross to claim his 30th league goal of the season, equalling his tally in 2012-13.
The last player to score more than 30 goals in a French top- flight season was Argentine Carlos Bianchi for Stade de Reims in 1975-76 when he bagged 34.
(Reporting by Julien Pretot; Editing by Ed Osmond)
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