Four hours is a half day of focused work or a braise. Split it in your head into two blocks of two, with a real stand-up between. This countdown will not insert the break for you. Stand up when you said you would.
Tell anyone who shares the kitchen or the lab what the alarm means so they do not turn it off to be helpful. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 4 hours countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 4 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Tell anyone who shares the kitchen or the lab what the alarm means so they do not turn it off to be helpful.
What is 4 hours actually good for?
Four hours is a half day of focused work or a braise. Split it in your head into two blocks of two, with a real stand-up between.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.