Four years? If we're talking a non Ti 780, than no, I don't think you'll receive that quality performance for that long. Maybe two years more or so, with that kind of quality. The way games are evolving now, it's going to become increasingly demanding for both GPU's and processors.
Is it still a beast card at the moment? Yes. Will it carry weight into the future? Yes. Will it have the same shelf that realistically a 970 has? No, not really. I wouldn't consider upgrading now though. There's really no point at all if you're playing with a single 1080p monitor. I'd save up casually over the next two years, and really drop some coin on the series following the 9xx. Or the series following the upcoming AMD response series.
That's where my money would be though