Ugh, the 85 percent crash loop is a special kind of hell. I remember pulling all-nighters back when I was running a rig that couldnt handle dynamic link workflows. It turned my timeline into a slideshow and honestly ruined my mood for weeks. I moved away from that platform because, unfortunately, the stability just wasnt there under heavy load, especially with 4K color grading. If you want to dodge the drama, you should really look into the AMD Ryzen 7 7700X 8-Core 4.5GHz Processor. It clocks super high and the architecture is way more efficient than what you are probably dealing with right now. I switched to a similar setup and the AVX-512 instruction support actually makes a massive difference in how Premiere handles those complex renders. Another solid route if you can find one on sale is the AMD Ryzen 9 7950X 16-Core 4.5GHz Processor, though that might push your budget a bit. I had issues with thermal throttling on my old board until I realized I needed a beefier cooler, so just make sure your current heatsink can handle the TDP. It is a total lifesaver when the render bar actually finishes instead of dying at the end. Dont bother with anything less than 8 cores for 4K stuff, honestly, or you will be right back in the same boat by next month.

TL;DR: Grab a AMD Ryzen 9 7900 12-Core 24-Thread Processor and call it a day. Its perfect for your budget. Man, I feel your pain. Rendering crashes at 85 percent are basically the worst way to spend a work week. Honestly, if youve got 500 quid, dont overcomplicate it. The AMD Ryzen 9 7900 12-Core 24-Thread Processor is gonna handle 4K without breaking a sweat, and it fits comfortably under your budget, leaving you some cash for a decent cooler if you dont have one already. Be careful with your motherboard though. Make sure you check if you need a new AM5 board, as that might eat into your 500 limit. If you really need to keep costs down, you might want to consider the AMD Ryzen 7 7700X 8-Core 16-Thread Processor. Its cheaper and honestly still flies through timeline scrubs. Just make sure your RAM is up to speed too, because 4K rendering is a total memory hog. Also, check your power supply before you swap. If you're currently running a low wattage unit, that could be why you're crashing in the first place, not just the CPU. Check your current temps while rendering, just in case you're thermal throttling. Sometimes its just the cooler struggling. Good luck getting that project out by Friday.