Elite Motorsport's Isaac Phelps set the pace on the first day of testing, with his best time coming in the third and final session of the day at the East Anglian circuit.
Phelps set a 1m48.206s with just over a minute remaining in the day's running, putting him 0.058s clear of teammate Ary Bansal, who also delivered his best effort in session three.
Douglas Motorsport's Luke Hilton and Hillspeed's Dan Guinchard had set the pace in the first two sessions of the day, with Hilton third overall courtesy of his morning effort.
The third Elite car of Alex Kattoulas completed a positive opening day for the local team, with his session three effort being 0.031s away from Hilton.
Dayton Coulthard was another to enjoy a strong opening day, finishing fifth in the final two sessions to keep that position on the combined times. He was 0.038s away from Kattoulas and 0.153s clear of Hillspeed's championship leader Dan Guinchard in sixth.
Alex O'Grady was the lead KMR Sport runner in seventh, 0.056s ahead of Fortec's Thomas Ingram Hill, who had been third quickest in the first two sessions.
Arden's three car line-up was headed by Leon Wilson in ninth, who was less than a tenth clear of Hillspeed's Leandro Juncos, who completed the day's top-10.
Arden's newcomer Dudley Ruddock ended the day in 20th place after improving his pace from session to session, while Flame Airikkala returned to GB4 action for the first time this year with a day's running for Pace Performance.
Testing resumes at the circuit tomorrow, with three more 30-minute sessions at 09.35, 13.15 and 17.00 respectively.
GB4 Championship, partnered by the BRDC, Snetterton Thursday test combined result:
1. Isaac Phelps, Elite Motorsport, 1m48.206s
2. Ary Bansal, Elite Motorsport, +0.058s
3. Luke Hilton, Douglas Motorsport, +0.860s
4. Alex Kattoulas, Elite Motorsport, +0.891s
5. Dayton Coulthard, Douglas Motorsport, +0.929s
6. Dan Guinchard, Hillspeed, +1.082s
7. Alex O'Grady, KMR Sport, +1.095s
8. Thomas Ingram Hill, Fortec Motorsports, +1.151s
9. Leon Wilson, Arden Motorsport, +1.443s
10. Leandro Juncos, Hillspeed, +1.539s
11. Callum Baxter, Graham Brunton Racing, +1.593s
12. Stefan Bostandjiev, Pace Performance, +1.626s
13. Jason Pribyl, ADM, +1.636s
14. Enzo Hallman, Douglas Motorsport, +1.725s
15. Jack Taylor, Fortec Motorsports, +1.761s
16. Alex Berg, Graham Brunton Racing, +1.761s
17. Ava Dobson, Arden Motorsport, +2.163s
18. Josh McLean, Pace Performance, +2.414s
19. Megan Bruce, KMR Sport, +2.415s
20. Dudley Ruddock, Arden Motorsport, +3.536s
21. Holly Miall, Fox Motorsport, +3.769s
22. Flame Airikkala, Pace Performance, +5.055s
23. Lily-May Watkins, KMR Sport, +5.371S
24. Caitlyn McDaniel, Fox Motorsport, +6.125s
