You can use contains to find the required letter. contains gives true if the part of search string contains in the main string.
x(contains(x(:,3),'a') & contains(x(:,4),'q'),(3:4))
Better to use strcmp, as it check for complete string. strcmp checks for the complete strings. It gives true only when the complete string is present.
x(strcmp(x(:,3),'a') & strcmp(x(:,4),'q'),(3:4))