Dawn raids took place at 12 properties in and around Barcelona on Tuesday, in an operation that began eight-months ago, said the Mossos d'Esquadra, the Catalan police force, in a statement.
ÚLTIMA HORA: Estem fent una operació antiterrorista en l'àmbit jihadista en diferents municipis de l'àrea metropolitana de Barcelona pic.twitter.com/8QvTXllTRq— Mossos (@mossos) April 25, 2017
It is not clear if the four were directly involved in carrying out the Brussels attacks, the spokesman added.
Four others were also detained over alleged links to jihadist organizations as well as organized crime activity.
Police said the suspects were Moroccans residents in Catalonia and aged between 31 and 39.
Belgium has been on high alert since March 22 last year when suicide bombers attacked Zaventem airport and Maalbeek metro station, killing 32 people and leaving more than 320 wounded.
The attacks were led by an Islamic State cell that was also responsible for the carnage in Paris in November 2015.
The men arrested in Catalonia "may have participated or cooperated in past actions" and may have travelled to Belgium, Catalonia's regional interior minister Jordi Jane told radio Rac1.
Some of the suspects are linked to organised crime groups and drug trafficking, he added.The latest arrests mean that more than 200 arrests for suspected jihadi activities have been made since Spain raised its national security alert to one step below maximum following attacks in France and elsewhere in 2015.