Crucial P3 4TB M.2 PCIe Gen3 NVMe Internal SSD - Up to 3500MB/s - CT4000P3SSD8 - £219.24 @ Amazon

No this is QLC NAND and DRAM Less, so bottom of the pile spec, anything worse would be a cheaper or similar priced Chinese cack brand. It's a very ordinary NVMe drive, it's not even PCIe 4.0. You're paying for the storage size.

Yes there is one scenario but its a rare one. The drive will use its empty space as a SLC cache (uses 4 times the amount of space but much quicker to write to).

If you exhaust the cache (you'd have to do a large write on a mostly full drive or a huge write on a empty drive) then the write speeds can drop to 100MB/s, far slower than your typical TLC SATA drive which will sit at 500+ MB/s all day. (edited)

Don't worry, Crucial have though of you...

... and released the P3 Plus with "PCIe 4.0".

Only problem is, it's the same Micron 176 layer QLC NAND, the same endurance and a tiny bit more performance. Just enough to it technically could no longer be PCIe 3.0 but which makes it one of the slowest PCIe 4.0 drives.

Not that headline burst rates count for much and I don't think Crucial / Micron even list the far more important IOPS metric.

Yes, they don't:
TBW at 800TB aka 200TB per 1TB is also pretty poor.

Unsure whether the P3 Plus is the slowest PCIe 4.0 drive as the comparison sites (PCPartpicker, Skinflint etc.) don't actually have a max speed filter for some reason! (edited)

£223 for the last three weeks.

If you're just reading games files, this is better. Much faster. If you're writing lots, the MX500 is slower, but will always deliver advertised speed - this will eventually drop to lower than SATA.

No, as another commenter said, the plus is D tier gen4