SaiPa had taken the series lead in game three in dramatic circumstances, surviving two disallowed goals which would have handed Lukko victory, before scoring in the second period of overtime.
Back on home ice in game four, they looked set for a less tense evening after Mikko Petman gave them the lead inside four minutes, deflecting Santari Airola's shot-pass past John Lethemon.
But top seeds Lukko hit back in the second period with a fine goal by Niklas Ylitalo, who evaded the defence and dragged the puck past Kari Piiroinen and into the net.
The lead was restored late in the second, as Lukko failed to clear from behind their own net and the puck was played to the arriving Mikko Kousa, whose slapshot was too strong for Lethemon.
A too many men on the ice call and a hooking penalty against Eric Gelinas either side of the break gave the home side a 5-on-3 situation early in the third period, which they took advantage of through Antti Kalapudas to put daylight between the teams.
However, Lukko showed their mettle when back to full strength as Pathrik Westerholm gobbled up a rebound to make it 3-2.
Once again, carelessness resulted in too many men on the ice for the visitors, and Juuso Heikkila's strike through a crowd meant SaiPa had made the most of another powerplay.
An empty-netter from Eero Klintrup rounded off the 5-2 victory on a great night for the Sputniks, who are now one win from a first ever appearance in the Liiga Finals.
Lukko must now win three consecutive games to stop them, and they will be back at home for game five tomorrow night.
There was only one goal in Kuopio, but that was all home side KalPa needed to open a two-match advantage over Ilves and move within one win of a first final since 2017.
KalPa had failed to find a way past Dominik Pavlat on Monday night, but this time it was Stefanos Lekkas' turn to record a memorable shutout, saving all 24 of the Tampere side's shots on target.
As with Pavlat in game three, Lekkas was the much busier of the two goaltenders this time around, making the clean sheet all the more impressive, as Ilves looked to build on their 3-0 home win in game three.
But this time, it was KalPa's turn to score against the run of play, doing so in the fourth minute of the third period.
Juuso Maenpaa kept the puck alive in the attacking zone, before unleashing a shot-pass which was turned home by Matyas Kantner.
The hosts survived a video review of the goal and then a late onslaught from the Lynx, but they held out for a 1-0 win.
Second seeds Ilves must also win three in a row to keep their season alive, and tomorrow will also be their first chance at home.
