Hillspeed's championship leader Dan Guinchard was in ominous form on the first day of action at Oulton Park, setting the fastest time in each of the three sessions at the picturesque Cheshire venue.
Having topped the first session of the day with a 1m35.120, he only just missed out on eclipsing that in the second session, recording a time just 0.068s away. But in the final session of the day, the British-Polish contender blasted to a 1m34.026s, lapping over four tenths of a second clear of the pack.
Elite Motorsport's Silverstone race winner Alex Kattoulas was his nearest rival with the second quickest time, 0.433s shy, in the final test, and 0.320s clear of the second Elite machine of Isaac Phelps.
Douglas Motorsport's Luke Hilton was fourth, with Guinchard's teammate Leandro Juncos cementing a spot in the top-five.
Fortec's Jack Taylor was fifth quickest in sessions one and two, but like the rest of the top-nine, recorded his best time in the session three to put him sixth overall for the day, 0.042s shy of Taylor.
Douglas Motorsport's Enzo Hallman was seventh quickest despite clipping the barriers in the final test, with Graham Brunton Racing's Alex Berg and KMR Sport's Alex Grady eighth and ninth respectively.
Berg's teammate Mayer Deonarine completed the top-10 thanks to his session two effort.
Testing resumes again tomorrow with two further sessions at 10.10 and 14.55.
GB4 Championship, partnered by the BRDC, Oulton Park Thursday combined test result:
1. Daniel Guinchard, Hillspeed, 1m34.026s
2. Alex Kattoulas, Elite Motorsport, +0.433s
3. Isaac Phelps, Elite Motorsport, +0.753s
4. Luke Hilton, Douglas Motorsport, +0.836s
5. Leandro Juncos, Hillspeed, +0.999s
6. Jack Taylor, Fortec Motorsports, +1.041s
7. Enzo Hallman, Douglas Motorsport, +1.547s
8. Alex Berg, Graham Brunton Racing, +1.582s
9. Alex O'Grady, KMR Sport, +1.596s
10. Mayer Deonarine, Graham Brunton Racing, +1.606s
11. Leon Wilson, Arden Motorsport, +1.610s
12. Luca Magnussen, Fortec Motorsports, +1.716s
13. Thomas Ingram Hill, Fortec Motorsports, +1.828s
14. Callum Baxter, Graham Brunton Racing, +1.854s
15. Ava Dobson, Arden Motorsport, +1.970s
16. Jason Pribyl, ADM, +1.999s
17. Stefan Bostandjiev, Pace Performance, +2.294s
18. Dayton Coulthard, Douglas Motorsport, +2.395s
19. Josh McLean, Pace Performance, +2.612s
20. Megan Bruce, KMR Sport, +3.154s
21. Holly Miall, Fox Motorsport, +3.512s
22. Lily-May Watkins, KMR Sport, +6.390s
23. Caitlyn McDaniel, Fox Motorsport, +6.549s