Fire crews attended the blaze at a block of flats on Westfield Road, Edinburgh, at around 12.30am this morning.
A Police Scotland spokeswoman said: “It appears it was a possible fire started deliberately.
“Four people were taken to hospital as a precaution with some minor smoke inhalation.
“The fire originated in the common stair of the block, which has 16 flats in the property.”
A Fire Scotland spokeswoman said: “Crews from Sighthill and Tollcross attended a fire in a cupboard in a common stair in a block of flats.
“We used one high-pressure hosereel and two firefighters with breathing apparatus to extinguish the fire, and a further firefighters with breathing apparatus for search and rescue.
“Three females and one male were taken to Edinburgh Royal Infirmary suffering slight smoke inhalation.”