how to configure fast DDR4 SDRAM for slower DDR4 controller on CPU

Question

I have some 4000MHz DDR4 SDRAM with stated timings of 20-20-20-40 that I want to install into a motherboard with a CPU that only supports 2000MHz DDR4 SDRAM (due to the memory controller).

Q1: If I just install this 4000MHz DDR4 SDRAM into the motherboard, will the BIOS be smart enough to automatically configure the memory controller (speed and timings) so: (A): the memory will work, and (B): the timings will be at the appropriate lower values to get maximum possible speed from the SDRAM (or close)?

Q2: What do the four timing values refer to?

Q3: If I configure the BIOS for these 4000MHz DDR4 SDRAM sticks with 20-20-20-40 timings to operate as 2000MHz DDR4 SDRAM sticks with 10-10-10-20 timings... are those values appropriate? More generally, how does one configure the BIOS to install faster DDR4 SDRAM into a motherboard (to get best possible performance)?

Answer 1

What happens if I use RAM of a faster speed than the CPU supports?

The RAM will be downclocked to match the FSB/Memory controller speeds. In this case that’d be 1600Mhz. So buying the faster rated memory is a waste of money unless you plan to change the CPU soon enough.

One of the comments

It's not always a waste of money. For example, memory rated for 2133 MHz at 1.65V would likely be able to run at 1600 MHz at a lower voltage, decreasing power usage and heat generated, and potentially increasing system stability.

Answer 2

The BIOS should handle it all for you automatically. Each DIMM contains a small ROM that contains its speed capabilities. The BIOS reads all of the installed memory and then configures the memory controller and DIMMs to run at the highest speed that everything supports.

The 4 timings are the minimum number of clock cycles that the controller must wait between different steps in the memory access cycle. Specifically they are the CAS latency, RAS to CAS delay, RAS precharge time and Row active times.
You will get different numbers for different clock speeds because internally the DRAM takes a fixed number of nanoseconds to do something rather than a fixed number of clocks, if you increase the clock speed then you need to wait more clock cycles for the same amount of time to have passed.

Generally yes you could configure the BIOS to use timings that are half the specified ones if you are running at half the clock speed. Or even better find out what the DIMM specifies at the clock speed you will be using, it probably will be half but it may not be for some reason.
However you shouldn't need to do this, as stated above the BIOS should be able to work it all out and pick the best speed anyway. Unless you are getting stability issues or are trying to overclock things it's normally best to let the system handle things.