I'm looking for a new computer custom or pre built. I work daily with blender (cycles) as a professional 3d artist but when it comes to technical things like picking a computer I feel lost. What should you get? my budget is limit is around USD 4000
I´m right now not up to date. So I can´t recommend parts. But what I can recommend you, is reading several articles about current hardware (this helps me always a lot). You get pretty fast an overview and can create a list. You can overview than your list and can optimize it. With the budget you should have some options.
You can also take a look on the requirements for Blender and can work a bit with that (for an overview): https://www.blender.org/download/requirements/
I would reccommend getting a ryzen 9 with 2/4*16GB of RAM, a 1/2TB ssd; X570 mobo, 1000W seasonic or corsair power supply, a case that looks nice and has been tested good by a youtuber/tech site, and a graphics card that fits the rest of your budget (nvidia is probably best, because of cuda support)