Elite Motorsport's championship leader Fred Green was the driver to beat on day one of the official mid-season test at Snetterton on Tuesday.
The British contender was quickest in the final two sessions, which switched from dry to wet weather running, with his session three effort of 1m50.017s being the best of the day.
That lap put him 0.149s ahead of fellow Elite runner Charlie Smith. The American karter has not yet competed in GB4, but adds a very positive Snetterton test to an encouraging run at Silverstone ahead of the opening round. His best time came by topping session two.
Fortec's Thomas Ingram Hill led the way in the opening session, and that time was enough to put the young Brit third overall on day one, ahead of Pace Performance's Torrin Byrne, who was just three thousandths of a second in arrears.
Hillspeed's Connor Willis was next up in fifth, 0.071s clear of Nitrous Competitions ADM Racing's Luke Hilton.
Elite's Matan Achituv set the seventh quickest time courtesy of his session three lap, 0.086s ahead of Fortec's Franciszek Cegielski in eighth, who was the top rookie contender.
Douglas Motorsport's Dayton Coulthard and Pace Performance's John O'Donnell completed the top-10, which was covered by 1.361s.
American driver William Rohan made an official GB4 test debut at the wheel of Enzo Hallman's regular Hillspeed machine, and was an encouraging 15th quickest. A total of 22 drivers participated today.
Testing resumes tomorrow, with four further sessions.
GB4 Championship, partnered by the BRDC, Snetterton official test, day one combined result:
1. Fred Green, Elite Motorsport, 1m50.017s
2. Charlie Smith, Elite Motorsport, +0.149s
3. Thomas Ingram Hill, Fortec Motorsport, +0.473s
4. Torrin Byrne, Pace Performance, +0.476s
5. Connor Willis, Hillspeed, +0.689s
6. Luke Hilton, Nitrous Competitions ADM Racing, +0.760s
7. Matan Achituv, Elite Motorsport, +1.031s
8. Franciszek Cegielski, Fortec Motorsports, +1.117s
9. Dayton Coulthard, Douglas Motorsport, +1.210s
10. John O'Donnell, Pace Performance, +1.361s
11. Joshua McLean, Pace Performance, +1.372s
12. Demitri Nolan, Hillspeed, +1.915s
13. Michael Koh, Arden Motorsport, +2.254s
14. Jordyn Martin, Fortec Motorsports, +2.293s
15. William Rohan, Hillspeed, +2.492s
16. Archie Bullard, Fox Motorsport, +2.538s
17. Jasser Iskander, Fox Motorsport, +2.645s
18. Lily-May Watkins, KMR Sport, +2.655s
19. Holly Miall, KMR Sport, +2.811s
20. Caitlyn McDaniel, Nitrous Competitions ADM Racing, +2.928s
21. Enzo Rujugiro, Fox Motorsport, +3.070s
22. Solenn Amrouche, Arden Motorsport, +3.149s