1. Can the previous PCIe Gen3 clk buffer product be used as the current Gen4 100MHz clk buffer?
2. What is the difference between PCIe Gen4 and Gen3 clk buffer? Any improve or any change? what should we pay attention about the new Gen4 product selection?
I would suggest LMK00334 (4-output) or LMK00338 (8-output) as PCIe clock buffers supporting up to Gen4. These buffers contribute very small additive jitter of 0.05 ps RMS to the PCIe clock source (from clock generator or oscillator).
The RefClk Max jitter limit is 1 ps RMS for Gen3 and 0.5 ps RMS for Gen4.
Please see this blog which should address your questions. The blog also mentions a couple PCIe clock generators from TI, in case you're interested in those as well.