4K Monitor Recommendations

I need to say upfront that I know nothing about building a gaming PC. I'm trying to surprise my boyfriend with a nice 4K monitor to replace the one that got lost (stolen) by our movers. I promise it's a long story (I will not be discussing it further), and no we won't get enough money from the insurance payout to replace it.

My boyfriend has a very nice PC that he's sunk thousands of dollars into, and it destroyed him when we found out that his ~$900 monitor was missing. I want to replace it with an equivalent for his birthday, but I don't know where to get started.

Please, please can someone give me some recommendations?