The world champion will now swap his rainbow bands for the ochre jersey on stage five – which concludes with a double ascent of Willunga Hill – with bonus seconds giving Sagan a two-second lead over Impey.
With searing heat the order of the day again, and temperatures in Adelaide well above the 40-degree mark, only two men braved a day in the breakaway – Zak Dempster and Alex Porter waving the UniSA-Australia flag.
They boasted a lead of nine minutes at one point, with the bunch keen to keep their powder dry, but the chase was timed perfectly.
As the heat from the peloton increased, former Rapha-Condor and Endura man Dempster left his compatriot behind but the fight for position on the run-in to Norton Summit put paid to the breakaway.
With BMC Racing controlling the tempo, race leader – and stage two winner – Caleb Ewan (Mitchelton-Scott) was dropped before they even reached the main climb.
More riders were then shelled out the back as four-time winner Simon Gerrans (BMC Racing) drove a furious pace.