The veteran racer suffered a high-speed off at The Chase right on the midway point of the final 40-minute session of the day.
The car made contact with the tyre barrier on the exit of the corner before coming to a rest on its roof.
Early reports are that Kassulke was communicating with rescue crews.
That proved to be the end of the session due to damage to the tyre bundles.
That left Rafaelle Marciello and the #999 GruppeM Mercedes on top of the times as Friday came to a premature close.
The pace was hot at the start of the session as the Pro drivers returned for the first time since Practice 1.
Marciello was straight into the fours on his first run with a 2m04.800s to lead the way.
Luca Stolz (#75 SunEnergy1 Racing Mercedes) and Mathieu Jaminet (#912 Manthey EMA Porsche) were initially second and third.
Right on the 10-minute mark there was a change at the top, Mattia Drudi going fastest in the sole all-Pro Audi with a 2m04.474s.
He didn't hold on to top spot for long, though, with Marciello and Stolz both immediately improving with a 2m03.995s and 2m04.136s respectively.
Jaminet was also able to improve shortly after that, easing the Porsche back into third with a 2m04.293s.
That was the final change near the top before the session was red-flagged.
The Pro Audi finished up fourth while Jordan Love (#24 Volante Rosso Mercedes) was the best of the Pro-Am entries in fifth.
The #10 International Motorsports Audi was best of the Silver entries in sixth ahead of the #888 Triple Eight Mercedes.
Next in line was the #777 Melbourne Performance Centre Audi, the #32 WRT BMW and the #44 Valmont Racing Mercedes.
Practice continues with a fifth session starting at 8:35am local time tomorrow.
Live coverage of the 2023 Bathurst 12 Hour will start on Motorsport.tv from Saturday onwards (not available to viewers in Australia and New Zealand).
