The characteristics provided in all the listed options (1,2,3 and 4) are indications of the Himalayas being young fold mountains, hence option 4 is the correct answer.

Option 1 considers only "Deep gorges" and "U-turn river courses". While these are indeed signs of young fold mountains, the absence of the other two elements makes it incomplete.

Option 2 includes "Deep gorges", "U-turn river courses", and "Steep gradients causing land-sliding". Again, these are signs of young fold mountains, but one key characteristic, "Parallel mountain ranges", is missing.

Option 3 suggests "Parallel mountain ranges" and "Steep gradients causing land-sliding". While these are indeed signs of young fold mountains, ignoring the first two makes it insufficient.

Option 4 includes "Deep gorges", "U-turn river courses", "Parallel mountain ranges", and "Steep gradients causing land-sliding", all of which are features of young fold mountains. Hence, it is the correct answer.