Excel cell relations

Hi 
i will give 4 cells numbers to explain what i want to do 
cell 1 is actual number , cell 2 is the target number , cell 3 is a value 'and can by changed' , cell 4 is the total number 'can not changed'
cell 4 = cell 1 +cell 3
i want to make cell 1 = cell 2 by changing cell 3 but cell 4 still the same