The Barcelona and Cambrils attackers had been planning bigger attacks than the ones they carried out, police have said.
Catalan police chief Josep Lluis Trapero told a press conference officers believed the group had been preparing the attacks at a home in Alcanar.
But an explosion in the house on Wednesday, which killed one person, wounded seven and destroyed material they were planning to use in the attacks, meant they had to carry them out in a "more rudimentary way", Mr Trapero said.
The four people arrested include three Moroccans and one Spanish national, police have said.
None of them had a criminal history relating to terrorism, police chief Josep Lluis Trapero told a press conference.
Three were arrested in the village of Ripoll and one in the town of Alcanar, where the house at the centre of the investigation exploded on Wednesday night.
The youngest person arrested was 21, while the others were 28, 34 and 27 years old, Mr Trapero said.