The first of two 15-minute sessions on Saturday morning was topped by Purdie in the #48 Audi R8 with a 1:09.8210s lap.
Kiwi young gun William Exton (#114 McLaren Artura) slotted into second, just 0.14s behind Purdie, with championship leader Tom Hayman (#25 McLaren) in third.
Blake Tracey (#35 Ford Mustang) continued the strong form shown in practice running fourth fastest, while lead Silver-Am competitor Zac Soutar was the third McLaren entry in the top five.
A pair of Ford Racing junior drivers were sixth and seventh, with American Sam Paley (#62 Mustang) closely followed by Cameron McLeod (#227 Mustang).
Shearer (#46 Porsche Cayman) was eighth, followed by Blake Dowdall (#95 Mustang) in the second Miedecke Motorsport entry.
Aaron Seton (#1 Mustang) rounded out the top 10 in the repaired Gomersall Motorsport entry.
Qualifying 1 ran unaffected for the entire duration, with no drivers finding trouble.
Two-driver entries swapped pilots for Qualifying 2, where Shearer set the pace with a 1:09.8598s to seal his first GT4 Australia pole position.
Shearer led home Jett Murray (#25 McLaren) by the narrowest of margins, with just 0.08s covering the pair.
Jobe Stewart rounded out the top three in the #48 Audi.
Several cars found trouble in the second session, including third-placed Stewart.
The Supercars rookie ran wide on the exit of Turn 1, taking light damage the the left-rear bodywork on the Audi.
Jack Johnson went sixth fastest despite a spin on the exit of Turn 1, while Nirwan found the grass at Turn 5.
Monochrome GT4 Australia returns for Round 5 at Sydney Motorsport Park on 19-20 September
