Ladder will reset on May 4th with the start of the new season. Typically, ladder resets occur 15 minutes prior to the start of the new ladder.
New ladders have consistently started at 5pm pacific time globally and will continue that way for Season 4.
Patch Notes will hit a few days prior to the new season along with the patch.
As for the start date, we are trying to build out some wiggle room for all Diablo games between D2/D3/D4 in terms of seasons so there isn’t overlap for players as many like to dive into varying games within the community.