Having retaken the series lead last time, Tappara made a brilliant start in game four in the hope of avoiding a repeat of their away defeat earlier in the series. Just 13 seconds in, Otto Rauhala diverted Henrik Haapala's pass into the path of Eetu Tuulola, who provided the simple finish.
It was 2-0 just three minutes later, after Petteri Nikkila was sent to the box for roughing, Tappara wore their opponents down on the powerplay, and Joachim Blichfeld's shot from the right found its way through to the net.
But HPK again proved their worth, getting back into the game soon after with a powerplay goal of their own following unsportsmanlike conduct from Aapeli Raasanen, and Viljami Jokirinne struck from just inside the blue line.
The key moment looked to have come in the final minute of the first period, when a nice pass from Benjamin Rautiainen set up Blichfeld for his second and gave the Battle Axe a two-goal cushion ahead of a goalless second period.
In fact, that's how it stayed until the final three minutes, when a real scrap for the puck in front of the Tappara net finally ended when Sami Paivarinta forced it over the line. Then, a minute later, an unlikely comeback was completed when HPK again put real pressure on the visitors' goal, and Olli Korhonen's effort found a way through to make it 3-3.
Yet, overtime belonged to the regular season winners, who took over 10 minutes to find the goal that secured victory, but patience was key, as that's just what Kristian Tanus used behind the net to wait for the right pass to become available, before finally finding a free Jesper Mattila, who fired the winner.
The 4-3 win on the night means Tappara have the chance to win the series at home in game five on Tuesday 7th April. HPK must win three games in a row to progress.
KooKoo had a much less stressful evening as they also took a two-game lead in their series against Pelicans, winning 5-1 on the road to give themselves three opportunities to reach the semi-finals.
Like Tappara, they wasted no time in going in front - Ville Meskanen won the puck in Pelicans' territory, danced through the defence and played the puck across for Veeti Miettinen to score with just 32 seconds played.
Midway through the match, the visitors doubled their lead in similar fashion, as this time Marcus Davidsson won the puck in Pelicans' territory and set the puck back for Vilmos Gallo to unleash a powerful finish.
As if twice wasn't enough, KooKoo again preyed upon the Lahti side early in period three, with Robert Rooba winning possession before he combined with Otto Paajanen to set up Ossi-Petteri Jaakola to add a third goal.
Pelicans' night got worse when they conceded a powerplay and KooKoo all but walked the puck into the net, with Miettinen adding his second goal. The home side did pull one back through Onni Lind soon after, but comeback hopes were long gone before Samuel Valkeejarvi found the empty net.
Back to Kouvola the sides go for game five on Tuesday 7th, KooKoo's first chance to wrap up the series victory, and therefore a must-win for the Lahti side.