The World No. 9 Indian, playing with black pieces, got off to a flying start and drew first blood as she took the Romanian's pawn in the third move of the game. But Deac ended up taking complete points from the game after a total of 78 moves.
"In the latter stages of the game I complicated the game with a move which was not needed and my position just kept getting worse," Harika said.
Harika has now moved down to joint third having won two, drawn one and lost one.
She will take on her good friend and compatriot Eesha Karavade in the fifth round of the tournament tomorrow and will play the sixth round in the evening.
Her opponent for her sixth-round clash will solely be dependent on her result against Eesha.