Some hdmi splitters don't work properly with 1080p even though they claim they do, I found I needed to ensure I got one that said works with PS4 - something to do with the hdmi spec. comes with a PSU but has a bright blue LED so needs a sticker to cover it
also, some of the cheaper ones only work on one output a time. the one i linked to can output to both at the same time.
hope it works for you