First off, my specs:
i7 2600k, 8Gb of RAM, Windows 7 64bit, EVGA 780 GTX, 120Gb SSD as the OS drive (BF4 is the only thing on the SSD other than the OS.)

Monitor 1 - Dell 2405 FPW (1920x1200 native)
Monitor 2 - ACER G215H (1920x1080 native)

In Battlefield 4 my friends are having no issues running the game in windowed fullscreen on one monitor, and the Battlescreen, mumble, friends list, shadowplay etc on a second monitor. Heck, some of them are watching youtube videos with no issues. They have lower specs than I do in the GPU department. I realize my CPU is a bit dated, but watching my CPU and RAM resources (logitech keyboard with the LCD output thing) while I'm gaming, it doesn't seem to get over 60% on either.

I run BF4 on a custom setting between medium and high, with the "really expensive" settings turned off.