I don't fully understand the request here - do you mean when the final render is going to be 1920 HD, you want IR to still be 4K? Because if the final render is 4K (or above) you can already resize the IR to show the 4K (or above). That said, if the final render is NOT going to be 4K, why do you want a 4K IR, when you can always zoom in e.g. using the mouse wheel, or just resize the IR window? Or just set the final render to 4K while working so you have a 4K IR, and then change the final render size when time comes for a final render?