Graphics card for LR/PS with 4k monitor

I recently picked up a 4k monitor (LG 27UD88W) and it has slowed my editing on my PC to a crawl. Making a simple change to a slider takes Lightroom several seconds to update and any major changes in Photoshop CC take minutes if not more. It is unmanageable.

Here are my PC specs:

Dell XPS 8700

i7-4790 (4th gen) w/ HD Graphics 4600

12GB DDR Ram

LR/PS installed on SSD Hard drive

Files stored on external 7200rpm HD

So is the gfx card the best upgrade here?  I was looking at a GTX 1060 or RX480 (never had an AMD card before), or possibly a used GTX 970 from someone I know (would the other two be a big upgrade over this?).

Machine is basically 100% used for photo editing.  No gaming of any kind.

Files are from A7r2 so 42mp, not counting stitched panoramas which I shoot fairly often and can sometimes get into the multiple GB size in Photoshop for the really big ones once I've added a bunch of layers and whatnot.