Max Record Length: 20K is a bit low these days.
How problematic is the max record length?
I don't know if it's "problematic", some time ago even "good" brands had 2.5k, but there are better options today for little money.
Since I'm currently experimenting with something like this, I'll show you with my $130 Hantek.
In the first picture there is a 20KHz sine wave at 10us/div, with a 50ns pulse every second (1Hz).
If you trigger on the pulse, a thin line appears in the middle. But there is nothing else to say about how the pulse looks. (1st picture).
If you stop it there and zoom in it to 50ns by turning s/div, you will see a very rudimentary pulse with 4k (2nd image). It doesn't tell you much about the shape of the pulse.
But if you stop it with 8M (record length), you can see much better in the magnification what is going on with the pulse (3rd image).
The more record length, the more accurate the magnification. Just like the resolution of a digital photo.
That is one difference. The experts here can probably explain this better, as I am a beginner too.
Btw. This is for illustration purposes only and is not intended to be a recommendation for the Hantek. It does not have 4 channels.