KMR Sport's Alex O'Grady posted the fastest time of the day as pre-event testing got underway at a sweltering Donington Park.
With ambient temperatures soaring well above 30° in the East Midlands, it was the Irishman's session one time which proved the best, as he registered a 1m29.769s.
Elite's Fred Green topped sessions two and three, while Hillspeed's Enzo Hallman got within a whisker in the final session, which put him second on the combined timesheets, just seven thousandths of a second in arrears.
Championship leader Green had to settle for third, with his session two lap proving his fastest, while Fortec's Thomas Ingram Hill was fourth thanks to his session one lap.
Nitrous Competitions ADM Racing's Luke Hilton was next up in fifth, with the top-five covered by less than three tenths of a second.
Pace Performance's Torrin Byrne set the sixth quickest time, 0.036s clear of Fortec's Franciszek Cegielski, with Hillspeed's Connor Willis eighth after improving in the final session.
Dayton Coulthard was sixth quickest in each of the first three sessions for Douglas Motorsport, but ended up ninth overall, ahead of Pace Performance's John O'Donnell.
The top-10 were covered by 0.651s at the end of the day, with the top-14 drivers within a second of the pace.
Testing continues tomorrow with four more sessions ahead of rounds seven, eight and nine of the season.
GB4 Championship, partnered by the BRDC, Thursday combined testing result:
1. Alex O'Grady, KMR Sport, 1m29.769s
2. Enzo Hallman, Hillspeed, +0.007s
3. Fred Green, Elite Motorsport, +0.048s
4. Thomas Ingram Hill, Fortec Motorsport, +0.184s
5. Luke Hilton, Nitrous Competitions ADM Racing, +0.297s
6. Torrin Byrne, Pace Performance, +0.380s
7. Franciszek Cegielski, Fortec Motorsports, +0.416s
8. Connor Willis, Hillspeed, +0.454s
9. Dayton Coulthard, Douglas Motorsport, +0.489s
10. John O'Donnell, Pace Performance, +0.651s
Click here for full result.
