In July 1915, Kostrba was transferred to Flik 4 on the Isonzo Front in northern Italy. He trained there as a pilot, qualifying in October 1915. He then served as both deputy commander and pilot in Flik 4, flying both Hansa-Brandenburg C.I two-seater reconnaissance planes and a Fokker Eindekker. On 18 February 1916, using Fokker Eindekker serial number 03.51, he scored three confirmed victories, though two were shared with other pilots.