Alright.
Ultra-Textures are not available yet, so the PC Gamer Vid only uses "High".
I doubt even the ultra-textures will use 6 Gigs on 1080p however - High uses 2.7 Gigs but with downsampling from 2720 x 1700 @ 1920 x 1200. Reason because I didn't try 1080 is because Mordor recognizes my downsampling-resolution as native and will only use percentages of that- so instead of 1080p I'll get some really weired ones like 1927 x 1187 or something like that (and I can't be arsed to fix it right now).
There is however a ingame-supersampling option so you can set the internal rendering to 200 % and will get effectivly 4K-Res while still being in 1080p. If I'd use that coming from 1080p instead of 1700p I'd wager you'd be around 3 Gigs. So the recommendation seems plausible.
So with Ultra-Textures taking somewhat more VRAM and Supersampling enabled, 4 Gigs might be just not enough, making the jump to 6 Gigs logical. - Remember: With Supersampling you're effectively running 4K not 1080p. Coming from there, I'd wager you'll be absolutely fine with 3 Gigs running this game with everything set to ultra, including textures but disabled supersamping.
It also runs very nice. Everything set to ultra except textures and downsampling I get an average framerate of just over 60 with some minor drops to ~50. Setting this to 4K, I still get 45+ with a R9 290X. This is an Nvidia-sponsored game, so Geforce-users will probably be at least slightly higher than this with a similar performing GPU (GTX 780- 780T i, Titan, GTX 970)
I'll test that more in-depth at some point next week, so we'll see.
So much for not opimized.
So don't get your panties in a bunch