Best monitor for game development and game art?

Well is it going to be an upgrade from my Asus VS239H-P monitors? I have 2 of these and looking to upgrade to either the Asus Rog Swift monitor or that LG 4K 60hz monitors. My budget is $700 maximum! I’m making a game inside of Unreal Engine 4 with old style PS1 graphics. Is refresh rate a big deal?