The t19 2pc will work with t20 4pc (or vice versa). This was confirmed by Ion as strategical choice for the players.
"There aren't plans to make the 2pc from different tier sets exclusive right now, but you will have to make a choice between legendary items, the 4pc bonus, and other 2pc set bonus. This allows for an interesting gearing choice, but if it ends up feeling like there is only one correct choice the team may revisit this."