Ignore @Valkyrie Lenneth, he's either trolling you or he doesn't know what he's talking about.
4 sticks is not always better.
Unless we're talking Threadripper or other very high end Intel processors, majority of processors on the market have a dual channel memory controller, where each memory channel can support a maximum of 2 memory sticks.
So, you get the most performance when you use at least one stick on each memory channel, so two memory sticks. If you install four memory sticks, you won't get better performance, it's still just two memory channels transferring data to the processor cores.
In fact, when you have 4 memory sticks installed, there's double the amount of signals going between the memory slots and the cpu socket which means there more 'noise' inside the circuit board and the memory controller inside the cpu has to work harder to receive the data and correct any small errors that may occur.
Because of this, you may find that on cheaper motherboards, you may not be able to run 4 memory sticks at high frequencies like 3200 Mhz, but the very same memory sticks would run just fine at 3000 Mhz - at lower frequencies each bit of information stays just a tiny bit more on the wires between cpu and socket and the memory controller inside the processor has a bit more time to decode these weak signals and detect correctly if it's a zero or a one bit.
So you should always install as many memory sticks as the amount of channels the memory controller inside the cpu has, or double the amount (but double that amount won't give you more performance).
For socket AM4, you have two memory channels. Threadripper has 4 memory channels, so you should install 4 sticks.
There's some Intel processors which have only 3 memory channels and in the past, AMD had socket AM1 or FM1 if I remember correctly which had only memory channel.
