I actually heard about this startup from a friend at work today (we're not from the US). Decided to post on HN after work (my evening is your early morning).
Ideally you want your press released at a time where it's most visible for your target demographic (unless it's bad news, in which case you release at the least visible time, a tactic that startups have been employing). Press releases aren't evergreen content.
So all 4 were released at the same time. I strongly question the use of a Early-Morning USA embargo.