Diogo - this may be a problem that is similar to http://www.mathworks.com/matlabcentral/answers/169197-nan-elements-during-table-data-input-matlab-gui. Your line of code
data = cell(edit1,4);
creates the cell array where each element is the empty array, []. If you try to populate any of these elements with strings, then you see NaN. Try instead to initialize this array to one of empty strings and reduce your code to simply
data = cell(edit1,4);
data(:) = {''};
% update the table
set(handles.uitable1,'Data',data);
I used the above code in a simple GUI, and was able to enter non-numeric data in any column (as well as numeric data which will still be considered as a string). Note that I removed the code to convert the array to a cell array (the call to mat2cell) since data is already a cell array.
