I thought since it was genitive it should be столов, but it is стола because it's four, right? 1 стол, ?2/?3/?4 (substitute ? with a digit, except when ? is 1) стола and 5+ столов (minus the previous case).
You're right. Same for feminine (одна дверь, две/три/четыре две́ри, пять/∞ двере́й) and neuter (одно о́блако, два/три/четыре о́блака, пять/∞ облако́в)
is stola the genitive plural?
It is the Genitive singular form.
The tip should have shown стола as an option. We are still learning when to use what cases with #s so duo shouldn't assume we know how it works all the time