Golden Road, 3 Strikes, and Triple Play are both considered "Big 3" due to their relative winability (1 in 24) and huge payouts (often upwards of $80,000). Similarly, they are not played very often so as to keep the budget down. Recently, Pay the Rent has been a 4th game that has similar properties (albeit slightly less winnable but can be easily manipulated so as to have a great shot at $10,000) but is not part of the original Big 3. Thus, "Big 4" would refer to all four, but "Big 3" excludes Pay the Rent.
Plinko has never bee considered a part of any of these categories since $50,000 is virtually impossible to win and it shows up on a relatively weekly basis.