How many grams of urea would have to be dissolved in 250 grams of water to give it a freezing point of -4.degrees celcius

Note the correct spelling of Celsius.

delta T = i*Kf*m
i for urea = 1
Kf for H2O = 1.52
You know delta T = 4.
Solve for m = molality

Then m = grams/kg solvent
You know kg solvent and m, solve for grams.