Based on the provided inheritance set, 'D' classes can be declared because class 'D' inherits from 'B' and 'C', both of which ultimately inherit from 'A', which is valid. Similarly, class 'Class_4(C,B)' is properly declared because it inherits from both 'C' and 'B', and both 'C' and 'B' inherit from 'A'. Therefore, the proper declarations are 'Class_1(D): pass' and 'Class_4(C,B): pass'.