You are passing in all of your data as one big long row vector (that is A1,...,A4 are all row vectors and you concatenate to a larger row vector with [A1,A2,A3,A4]), but boxplot treats different columns as different variables, or in the case of a row vector it will treat this as many observations from a single variable. This is fine (excepting that it does not have the results you want) but you then provide 4 labels and thus the error. For this reason you need to pass in a grouping variable as well, which will describe which group the data came from:
G = [ones(size(A1)) 2*ones(size(A2)) 3*ones(size(A2)) 4*ones(size(A2))];
X = [A1, A2, A3, A4];
boxplot(X,G,'notch','on','colors',[0 0 0],'symbol','','labels'{'data1','data2','data3','data4'});
Note, if you want to get rid of the outliers you want to set the symbol to the empty string as opposed to setting the outliersize to 0.
