About waking at 4:30 AM
Half four is the classic bakery, farm and early-shift start. If you are setting it for the first time, note that the last sleep cycle before waking is the one richest in REM sleep, so cutting the night short here costs disproportionately more than the hour it looks like.
Leave the tab open
A browser alarm can only ring while its page is open. It does not have to be the tab you are looking at, and on a desktop it does not have to be the window in front — but closing the tab or quitting the browser stops it. For anything you genuinely cannot miss, set a second alarm on your phone as a backup. That is true of every browser-based alarm clock, not just this one.