PCIe 4.0 x4 is capable of 64 Gbps in each direction.
Correct! The raw data stream of O's and 1's (binary digits)
transmits 16 Billion such binary digits per second
over a single serial channel.
Just multiply by the number of serial channels
to calculate maximum feasible throughput.
Where some people stumble is the conversion
to "bytes per second". This is easily done by
understanding the "jumbo frame" layout aka 128b/130b.
The latter refers to 16 x 8-bit bytes + 1 start bit + 1 stop bit
for a total of 130 binary digits per jumbo frame:
16 x 8 + 1 + 1 = 128 + 2 = 130 bits
Thus, the exact divisor is 130 bits / 16 bytes = 8.125 bits per useful byte.
And, 16 Gbps / 8.125 = 1,969.23 Megabytes of useful data per x1 PCIe 4.0 lane.
(The latest USB standard uses a slightly different jumbo frame: 128b/132b .)
Hope this helps.