The Audi Sport Abt driver posted a 1:29.920s effort shaving 0.3s off the previous unofficial record time set by team-mate Daniel Abt on Monday.
Di Grassi's time was set while utilising the 200kW qualifying mode as he topped the timesheets by 0.4s from nearest rival Sebastien Buemi in the e.dams machine.
“I have to say that going for the time is not as important as working on the reliability of the car,” said di Grassi.
“Of course working on the performance is important, but we know that performance here doesn't necessarily translate to what we will see at the other tracks.
“I think the lap time shows promise and it's good to know that our package is running well at 200kW.”
Mahindra enjoyed a strong day with Bruno Senna third fastest ahead of new signing Nick Heidfeld, with the pair 0.7 adrift of di Grassi's pace.
Nicolas Prost was just three thousandths of a second shy in fifth, while DS Virgin Racing's Jean-Eric Vergne ended the day in sixth.
Jerome D'Ambrosio clocked up the most mileage as he logged 71 laps behind the wheel of the sole Dragon Racing entry.
Trulli GP encountered further issues with its new Motomatica JT-01 machine.
A final two-day test will be held from August 24-25 ahead of the season opener on the streets of Beijing from October 17.
Result: FIA Formula E Pre-Season Test Day 4, Donington Park
|Lucas di Grassi
|DS Virgin Racing
|Nelson Piquet Jr
|Simona de Silvestro