Checkin is at 4. VIP Owner checkin starts around 2pm.
Officially.
Unofficially, i've shown up pretty early before and manged to check in. It really depends on a lot of factors such as your desire for a particular location within the resort, the amount of turnover at the resort that day, etc.
Worst case, You can always see about checking in and doing the paperwork early, and if the room isn't ready, just drop your bags off at the bell services desk and either enjoy the resort amenities or head to a park or DTD until the room is ready later in the day.