This achievement will hurt you. Physically.*
As of 9/17/2021, 4-Key Sprint has 64 notes that you have to complete perfectly to get the achievement. In order to do this, you will need to
correctly press 8 notes a second. This means that half of the notes you press every second will be behind and unreadable until you press the first note in an 8-note sequence.
Every .5 seconds. Yeah. I know.
This is doable (there's ~110 people in the world at this writing that have managed to do it) but it's going to take a couple days of practice and some serious muscle memory. The Planetary Piano timer is going to be your metric of success. The clock starts at 15 seconds and counts down, meaning you'll have to have >7 seconds left on the clock by the time you finish.
Start slowly, start getting your accuracy up, and try to get consistently in the 6 seconds left range. Repeated notes are going to be your enemy, since arpeggiated runs are easier to spam. 90% of it is getting a good mapping, but since there's 10 songs and 4 different maps per song... so it's very unreliable. As a general rule,
Bach Prelude and Fugue No.1 and Vivaldi's
Storm seem to have more arpeggios than repeated notes, so they are more reliable in my experience.
Once you're around the 6.5 second mark consistently, just let go of your caution and start flying through songs as fast as you can. That last .5 seconds is the hardest to make up, and you're only going to be able to do it through muscle memory and sightreading. Eventually, if you go hard enough and have enough maps under your belt, you can probably squeeze out the achievement on a lucky RNG mapping.
*
A safety note: Carpal tunnel is actually a danger if this achievement takes too long. Especially with repeated notes, the human wrist is not expected to push out 8 of the same key every second for 8 seconds. To that end, while I was grinding I got some tips on care from druid07 - a planetary piano speedrunner who's around #50 last time I checked:As someone who plays a lot of osu, carpal tunnel is a real worry. Take a break if burning feelings are sneaking up on you in your wrists. If you still hurt, sign language exercises can help.