As was mentioned in the other thread, it gives a lot more than exactly the same but at 4K. Basically 90% of games are improved in some way. Simply using a 4K pipeline outputting to a 1080p screen looks considerably better. I don't have a 4K TV, bought full price with no trade in even and it's been easily worth it. I do have a PSVR though. Imo it's a no-brainer for ~$100 more, less than the price of 2 games.
This thread has details on a per-game basis:
http://www.neogaf.com/forum/showthread.php?t=1324251
"Next 5 years" is also quite a long timeframe to commit to not buying any 4K displays.