The Australian's win came hours after police said they foiled a plot to spill 400 litres of a liquid similar to motor oil on the road during yesterday's action.

The quartet was arrested on Saturday and are thought to be part of a group advocating for Catalonian independence.

They were then released today and ordered to stay at least 500 metres away from the race.

Authorities said they had found two barrels full of oil, which were connected to a pipe leading to a bridge above the planned route.

Alpecin-Deuninck's Groves won stage four by a whisker with a time of 4hour 5min 41sec after an early breakaway was reeled in before the line.

Colombian Sebastian Molano of UAE Team Emirates and Belgian Lidl-Trek rider Edward Theuns followed him across the line in Tarragona following a gruelling 183.4-kilometre stage that began in Andorra la Vella.

"Sebas [Molano] went full from the bottom with 350 metres to go and I was luckily patient enough and strong enough to close the gap with around 100 metres to go," said Groves, who

"I was confident I could pass him because he led out from very early.

"On a finish like that, which is super tough and long, I thought he would have to be on a really good day to beat me."

There was no change to the top of the general classification standings as defending champion Remco Evenepoel of Belgium and Soudal Quick-Step, who won stage three, still holds the red jersey.

He has a five-second lead over Movistar's Enric Mas of Spain while Groupama-FDJ Frenchman Lenny Martinez occupies third place.

The race is set to continue tomorrow with another hilly stage that takes riders from Morella to Burriana over 185.7km.