Added part 4 materials

Part 4.1 - Object Properties.md

Part 4.2 - Data Structures and Algorithms.md

Part 4.3 - Data types and variance.md

Part 4.4 - Generics.md

Part 4.5 - Generics and Variance.md

images/part-13/eq1.PNG

images/part-13/eq2.PNG

images/part-13/eq3.PNG

images/part-13/eq4.PNG

images/part-13/equivalence.PNG

images/part-13/hash.PNG

images/part-14/Collection_hierarchies.PNG

images/part-14/Collection_hierarchies2.PNG

images/part-14/Collection_types.PNG

images/part-14/Collection_types2_inverted.PNG

images/part-14/Properties_of_collections.PNG

images/part-14/threadprogramming.PNG

images/part-15/invariance.PNG

images/part-16/generics.PNG

images/part-17/hierarchy1.PNG