The Sun radiates 3.8 × 10²⁶ W. Find the mass that is turned into energy each second and in one year…

Step 1. Each second. $\dfrac{P}{c^2} = \dfrac{3.8 \times 10^{26}\ \text{W}}{(2.998 \times 10^{8}\ \text{m/s})^2} = 4.23 \times 10^{9}$ kg.

Step 2. In a year. $4.23 \times 10^{9}\ \text{kg/s} \times 3.156 \times 10^{7}\ \text{s} = 1.33 \times 10^{17}$ kg.

Step 3. The share. The mass before is $4 \times 1.007825\ \text{u} = 4.031300$ u. The mass lost is $4.031300\ \text{u} - 4.002603\ \text{u} = 0.028697$ u. So the share is $\dfrac{0.028697}{4.031300} = 0.00712$, which is $0.712\ \%$.