Alvaro Garcia Ortiz, the regional prosecutor, said the four were detained for questioning in the northwest region of Galicia.
On Tuesday, 64 fires were still active across the region and five were burning out of control, leading to evacuations, fire department officials said.
High, gusting winds were making it difficult to put out fires, an official said.
Most of the outbreaks had been near urban areas close to the coast, said Garcia Ortiz.
The drought conditions that have prevailed for the last five years have made firefighting difficult, Garcia Ortiz said.
Galicia has suffered an average of about 12,000 forest fires per year recently, said Garcia Ortiz.
Nine out of 10 fires were started deliberately, but few arrests have lead to prison sentences, said Garcia Ortiz.
Many factors cause people to want to start forest fires deliberately, said Garcia Ortiz, including people wanting to re-zone land for new homes or to turn forest into grazing land, said Garcia Ortiz.
One man was arrested in Cerdedo, a town in Pontevedra, after authorities acted on tips from local residents, the Galician regional government said in a statement.
Almost 5,000 hectares have been
charred in the Galicia region
Almost 5,000 hectares have been charred in Galicia in this year's fires.
Seven wildfires also were reported on the other side of Spain in the northeast region of Catalonia where high winds also hampered firefighting.
Forest fires in Spain and other Mediterranean countries char hundreds of thousands of acres of land every year. Spain's national and regional governments agreed to become more vigilant after 17 people died in fires last summer.
The Environment Ministry has been trying to reduce the number of forest fires that affect Spain each year.
The number of highly destructive fires - ones that have burned more than 1 hectare - has dropped from more than 6,200 in 2005 to under 3,000 in 2006 for the January-July period, according to ministry figures.
In the same period fires have charred 35,870 hectares in 2006, down from 94,450 hectares in 2005, the ministry added.