The fire broke out at the Svyato-Pokrovska mine in the Donetsk region at 11:44 am, the Donetsk administration said in a statement.
Twelve of the 28 miners working at the time were in an area directly affected by the fire.
"According to preliminary information, four miners were fatally injured," the statement said.
Deadly accidents are frequent at Ukrainian mines, most of which are located in the country's industrial east.
Many mines are poorly equipped and maintained, and safety violations are frequent.