The Ukrainian army said it had been hit by rebel fire in the battle for control of the key industrial town of Avdiivka.
The outskirts of the Kiev-held steel town close to the rebels' de facto capital Donetsk has in recent months seen some of the fiercest clashes in the 33-month war.
"Unfortunately four of are troops are dead and five others injured," the army said on Facebook.
"Today, the enemy attempted to seize the Avdiivka industrial zone," Kiev military spokesman Oleksandr Motuzyanyk said earlier, adding that the firefight involved artillery and large-calibre mortars.
More From This Section
The fatalities came despite a ceasefire announced in December.
About 10,000 people have died since the start of a pro-Russian rebellion in 2014 that Kiev and the West accuse Moscow of masterminding.
Fighting has dragged on and neither side is willing to make concessions in the conflict that has pushed ties between Moscow and the West to their lowest point since the Cold War.