4k is not happening under $1k. At least for games like RDR2. I'm assuming you still have your older system with the GTX 970 and the Corsair micro ATX case plus your PSU, drives, and SSD, right? In that case I'd go something like:
PCPartPicker Part List
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor ($174.99 @ Amazon)
Motherboard: ASRock X570M Pro4 Micro ATX AM4 Motherboard ($179.99 @ Amazon)
Memory: G.Skill Ripjaws V 16 GB (2 x 8 GB) DDR4-3600 Memory ($74.99 @ Newegg)
Video Card: EVGA GeForce RTX 2070 SUPER 8 GB XC ULTRA GAMING Video Card ($503.98 @ Newegg)
Total: $933.95
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-03-12 01:43 EDT-0400
I usually hate recommending expensive boards but I would absolutely want something capable of running PCIE-4.0 SSD if I'm building today, so X570 is a must, and your X570 choices are super limited in the micro ATX form factor. But if you're not worried about that you could go with a cheaper B450 board and either upgrade the cpu to a Ryzen 7 3700X or the gpu to an RTX 2080 Super and still stay in budget. RTX 2070 Super will play a lot of current AAA games well at 4k (it's a little more powerful than the 1080 Ti), but it's a bit weaker than what has been revealed for the new XBox Series X.
Also AMD is the way to go on cpu right now. Intel ing sucks unless you're able to spend $550 on an i9-9900k. Otherwise AMD's cpus are hands down better. And they're cheaper too.