Heat absorbed by water: Q=mcΔT=4×4200×16=268800 J.
Volume change: V1=4/1000=0.004 m³ at 4°C, V2=4/998≈0.00401 m³ at 20°C.
Work done by water at constant pressure: W=PatmΔV=105×0.00001=0.8 J.
By first law of thermodynamics: ΔU=Q−W=268800−0.8=268799.2 J