The German sprinter claimed his second stage win of the week for Lotto-Belisol as he stayed out of trouble in an eventful sprint finish featuring two separate crashes. Roberto Ferrari (Lampre) and Jonathan Cantwell (Team Saxo-Tinkoff) were second and third on the day.
Welshman Thomas was also safe from harm and remains ahead of Blanco's Tom Jelte Slagter in the overall standings as he bids to bring home the title for Team Sky.
He said: "It was a stressful day as there was a bit of wind and I think everyone was thinking about the potential cross-wind sections.
"The boys were great. We stayed near the front with about 10 kilometres to go and they kept me out of trouble."
Saturday's pivotal fifth stage covers 151.5 kilometres, with three laps of a circuit starting from McLaren Vale before the final climb of Old Willunga Hill, with the finish carrying a 10-second time bonus.