The latest bomb - which was placed in a Renault Clio car - went off in the northern city of Santander.
It exploded at 1445 local time (1345 BST) on Saturday, causing damage to a nearby building, local newspaper reports said.
A 33-year-old tourist from north London remains seriously ill in hospital after being hit in the chest by shrapnel in the first blast which hit the Costa Del Sol area.
The man was one of six people injured when the bomb exploded on Friday outside Las Piramides - a hotel popular with the British - in Fuengirola.
He underwent emergency surgery at the Costa Del Sol Hospital in Marbella and is said to remain in "serious" condition.
Two other British children, a Moroccan and a Spanish couple were slightly injured.
A total of three bombs went off on Friday - in Fuengirola, Marbella and Saragoza. The fourth exploded at 1305 local time on Saturday (1205 BST).
No one was injured in the blast, which happened in a hotel car park between the beach resorts of Fuengirola and Mijas, damaging two cars.
Terror warning
The bombings have all been blamed on the Basque separatist group ETA.
As a result of the explosions, the UK Foreign and Commonwealth Office updated its advice to Britons travelling to the area, warning them to be alert to "continuing terrorist activity".
A spokesman said on Saturday: "The earlier warning still stands."
Tourism chiefs expressed anger at the blasts and said that UK holiday companies were on a state of alert.
But the Association of British Travel Agents said British tourists in the area appeared happy to stay on despite the attacks.
A spokeswoman for the Costa Del Sol Hospital had said the condition of the British man injured in the Fuengirola blast was "still serious", but she added he was "out of danger and recuperating in intensive care".
She said his family was flying out to Spain on Saturday to be at his bedside.
Summit
The Fuengirola explosion happened when a grey Peugeot 205 exploded shortly after 0700 local time (0600 BST), while many in the resort were asleep in their hotels.
Others were already gathering in bars to watch England play Brazil in the World Cup.
The second blast in Marbella was at 1305 local time, six hours after the first, but there were no reports of injuries.
The third, in Zaragoza, happened at 2215, local time (2115 BST). A security guard and a passing French national sustained minor injuries in the explosion in the car park of the Corte Ingles department store.
The attacks took place as European leaders, including Prime Minister Tony Blair, gathered for an EU Summit in Seville, southern Spain.