This reminded me of the 4 minute mile. The first 4 minute mile was a 3:59 run by Roger Bannister in 1954. Prior to that, the record was a 4:01 run in 1945.

A 4% improvement on the 4:01 would be a 3:51. A 3:51 wasn't achieved until 1966, about 20 years later.

So 4% can be quite a jump!

To put that into context, Bolt's 9.58s (current WR, considered phenomenal in and of itself, I think?) is about 1.7% faster than Powell's 9.74s (last pre-Bolt WR).

A 4% improvement on Bolt's 9.58s would be about 9.19s - probably not something we'll see for several decades, if ever.