The 30-year-old Australian successfully defended her title from last year, and becomes the first rider in the race's history to won two editions.
Spratt finished in the bunch as fellow Australian Chloe Hosking (Alé-Cipollini) took the bunch sprint win ahead of Giorgia Bronzini (Cylance Pro Cycling) in second and stage one winner Annette Edmondson (Wiggle-High5) in third. British rider Abigail Van Twisk (Trek-Drops) finished in fourth.
Spratt had taken the race lead after Saturday's hilly third stage, winning ahead of American Lauren Stephens (Cylance Pro Cycling) in Hahndorf.
It wasn't all plain sailing for Spratt on the short 46-kilometre final day of racing. A crash mid-way through the race saw her take a tumble with a number of other riders, and she took a lap out to rejoin the bunch on its next pass of the circuit.
Spratt finished the four-day race 29 seconds ahead of second-placed Stephens, with Katrin Garfoot (UniSA-Australia) in third at one minute and 21 seconds.
Emma Grant (Tibco-Silicon Valley Bank) was the highest-placed British rider overall in 10th, 2-46 adrift of Spratt.
With the Women's Tour Down Under concluded, the men's racing kicks off with the People's Choice Classic criterium. The men's Tour Down Under then starts on Tuesday, January 16 – the opening race in the 2018 UCI WorldTour.
Women's Tour Down Under 2018, stage four: Wakefield Road to Wakefield Road, 46km
1. Chloe Hosking (Aus) Alé Cipollini, in 1-07-29
2. Giorgia Bronzini (Ita) Cylance Pro Cycling
3. Annette Wdmondson (Aus) Wiggle-High5
4. Abigail Van Twisk (GBr) Trek-Drops
5. Maria Vittoria Sperotto (Ita) Bepink
6. Emilie Moberg (Nor) Virtu Cycling
7. Jelena Eric (Srb) Cylance Pro Cycling
8. Sarah Roy (Aus) Mitchelton-Scott
9. Alison Jackson (Can) Tibco-Silicon Valley Bank
10. Marlies Mejías (Cub) Sho-Air Twenty20
Final general classification
1. Amanda Spratt (Aus) Mitchelton-Scott, in 10-47-21
2. Lauren Stephens (USA) Cylance Pro Cycling, at 29 secs
3. Katrin Garfoot (Aus) UniSA-Australia, at 1-21
4. Lucy Kennedy (Aus) Mitchelton-Scott, at 1-36
5. Grace Brown (Aus) Holden Team Gusto Racing, at 1-54
6. Annemiek Van Vleuten (Ned) Mitchelton-Scott, at 2-07
7. Shannon Malseed (Aus) Team Tibco-Silicon Valley Bank, at 2-09
8. Kate McIlroy (NZl) Specialized Women’s Racing, at 2-25
9. Grace Anderson (NZl) Vantage New Zealand National Tea, at 2-42
10. Emma Grant (GBr) Team Tibco-Silicon Valley Bank, at 2-46
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