Why 4 minutes
Four minutes is a complete Tabata block: eight rounds of twenty seconds of maximal work and ten seconds of rest. It is also the traditional benchmark for the mile, and roughly how long a French press should sit before you push the plunger down.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 4 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.