% Creating random sample data
finalImageAll{1}=rand(3,3);
finalImageAll{2}=rand(3,3);
finalImageAll{3}=rand(3,3);
finalImageAll{4}=rand(3,3);
% now summing them all up in one line and no loop
finalImage = sum(cell2mat(reshape(finalImageAll,1,1,numel(finalImageAll))),3)
And the results: (it is random numbers so you would get something else)
finalImage =
2.9640 2.6123 1.1453
2.6784 1.9987 1.8235
1.9245 2.1087 3.4398