We need to calculate the difference in mass numbers and atomic numbers.
Mass number of U = 238
Sum of mass number of products Sr and Xe = 95 + 139 = 234
Atomic number of U = 92
Sum of atomic numbers of products Sr and Xe = 38 + 54 = 92
Atomic number is same, meaning that no protons are emitted. Since the mass number decreases by 4 and no protons are given off, 4 neutrons must have been emitted.