I tried this right now and noticed that I count tap for the first time. But the most important thing is that all of them are different and form a repeatable pattern. So brain just remembers the sensation of the fourth tap and I can tap in fourths subconsciously for a while.

The other way to deal with this exercise is just tapping for the odd count in one place and for the even in another. So you know when the fourth tap is because you subconsciously know it was long enough time after the second one and it is in the right place on the table.