In 1914, Christian communities in Turkey numbered around 4.5 million and represented 15%-20% of the population
72% confidence
EqualityTurkey
Omissions
The claim does not specify whether the 4.5 million figure refers to the entire Ottoman Empire or specifically to the region of modern-day Turkey (Anatolia). The 4.5 million figure appears to apply specifically to Anatolia/Asian Turkey, while the Christian population of the entire Ottoman Empire was lower (around 3.3-3.6 million).
The demographic data from 1914 Ottoman censuses is subject to historical debate due to methodological inconsistencies and political factors.
The sources found are secondary (Wikipedia, academic websites) rather than primary Ottoman census documents.