You would still need the fastest possible memory modules.
Not necessarily, you could end up with 3GHz RAM performing better than 4GHz if you have to run a 1/2 divider, with a 1/2 divider IF would be running at 1000Mhz with 4GHz RAM whereas 3GHz would be using a 1/1 divider so would be running at 1500MHz.
Ryzen doesn't benefit from higher clocking RAM because the RAM is faster, it benefits because the MemCLK and IFCLK share the same clock domain so higher clocked RAM automatically clocks IF higher.