What time do you send out the check-in message?

We send out the message at 2 pm. We explicitly state in the first sentence, check-in is not until 4pm. We are starting to run into an issue where guests are showing up early (1 to 2 hours) without permission. The issue worsens when they contact Airbnb claiming an uncleaned home if they arrive before the cleaner or while they are finishing up and Airbnb, with no investigation, cancels and gives them a full refund while also blocking off the dates. It's getting frustrating dealing with support especially when they have already taken action.

Should I just send out the check in message at 3 or right at 4?