In googling the specs for 7870, it has some form of these for video outputs...
1x HDMI 1.4a
2x mini-DisplayPort 1.2
Assuming your exact model is the same (should be), HDMI 1.4a can do 4K but can only up to 25Hz. It seems like you wouldn't be able to test 4K/120 using that card. IIRC, HDMI 2.1 is what brings 4K/120 to the standard. DP 1.2 doesn't support it either (1.4 does).