A search for "Call of Duty Black Ops 4 APK + OBB" yields two primary categories of results. Both are problematic:
Unlike Call of Duty: Mobile (launched in 2019), which was built from the ground up for touchscreens, BO4 was designed exclusively for PlayStation 4, Xbox One, and PC. Call Of Duty Black Ops 4 Apk - Obb For Android
Enable "Install from unknown sources" (Settings > Security > Unknown Sources). A search for "Call of Duty Black Ops