390x is your best bet right now. 980 might work i know i run skyrim maxxed with insane ENBs and grass on my 780ti at 40 fps and it sits at 2.9gb vram usage. that is at 1080p though, Your frame ddrops are most caused by your card just not being powerful enough not just the Vram,
Also NOTHING for at least 2 years will run modded skyrim at 4k. It just isnt gonna happen