Elite Motorsport's Isaac Phelps topped Friday testing at a soaking wet Donington Park, with rain falling constantly throughout the day ahead of the deciding rounds this weekend.
With conditions worsening throughout the day, it was the first session right at the start of the day that proved quickest, and Phelps topped it with a 1m40.092s.
The British driver, who enters the weekend second in the standings, was over half a second quicker than the rest of the field, with Arden Motorsport's Leon Wilson his nearest challenger.
Hillspeed's Dan Guinchard, who topped session two, was third, a further half second further back, with Fortec's Jack Taylor, who has an outside title chance, fourth and just a tenth further back.
ADM's Jason Pribyl enjoyed a positive day with the fifth quickest time, three tenths clear of the second Hillspeed of Leandro Juncos, with Fortec's Luca Magnussen a quarter of a second further back.
Pace Performance's Stefan Bostandjiev was eighth fastest, 0.021s ahead of Fortec's Thomas Ingram Hill, while Elite's championship fighter Ary Bansal completed the top-10, and topped the third and final session.
The GB4 field returns to action in qualifying tomorrow (Saturday) at 11.35.
GB4 Championship, partnered by the BRDC, Donington Park, Friday test combined result:
1. Isaac Phelps, Elite Motorsport, 1m40.092s
2. Leon Wilson, Arden Motorsport, +0.585s
3. Dan Guinchard, Hillspeed, +1.164s
4. Jack Taylor, Fortec Motorsports, +1.283s
5. Jason Pribyl, ADM, +1.504s
6. Leandro Juncos, Hillspeed, +1.810s
7. Luca Magnussen, Fortec Motorsports, +2.061s
8. Stefan Bostandjiev, Pace Performance, +2.122s
9. Thomas Ingram Hill, Fortec Motorsports, +2.143s
10. Ary Bansal, Elite Motorsport, +2.256s
11. Ava Dobson, Arden Motorsport, +2.257s
12. Dayton Coulthard, Douglas Motorsport, +2.393s
13. Lucas Blakeley, Douglas Motorsport, +2.739s
14. Alex Kattoulas, Elite Motorsport, +2.924s
15. John O'Donnell, Pace Performance, +3.628s
16. Enzo Hallman, Douglas Motorsport, +3.652s
17. Josh McLean, Pace Performance, +3.883s
18. Dudley Ruddock, Arden Motorsport, +4.149s
19. Alex O'Grady, KMR Sport, +4.309s
20. Lewis Wherrell, KMR Sport, +4.612s
21. Flame Airikkala, Hillspeed, +4.978s
22. Holly Miall, Fox Motorsport, +6.815s
23. Lily May Watkins, KMR Sport, +7.771s
24. Caitlyn McDaniel, Fox Motorsport, +9.060s
