Complex sentence: Three of the four ships were launched in 1915 and the fourth in 1916, but work on the gun turrets lagged, and it became evident that Russian industry would not be able to complete the ships during the war.
Split into simpler sentences: Three of the four ships were launched in 1915 and the fourth in 1916. Work on the gun turrets lagged, and it became evident that Russian industry would not be able to complete the ships during the war.