If I put a Gen4 with 6000MB/s max speed into a Gen3 slot it means the max speed will be ~3000MB/s?
I would expect it to be ~3500 which is typically what Gen3 drives top out at.
It’s a data transfer bottleneck not a limitation directly on the drive itself. Additional speed capability of the drive will be wasted in a 3.0 slot. Don’t think of it as the drive’s data speed, it’s your cpu/mobo speed.
IMO, look at both 3.0 and 4.0 drives and pick one with the capabilities you are looking for, there is a good deal of overlap between the high end 3.0 and low end 4.0.
It’s not a good leapfrog piece of hardware, 5.0 is out now and 4.0s has/will shortly finish maturing and further drop in price- if you actually need more speed at some point, buy then, imo, assuming this new pc is not happening within the next 6 months or so. If it is farther out you don’t know if 5.0 will be a consideration or not.