You should have plenty of time to make the 4:15 ferry. As mentioned just take the next one that is leaving out of Charlotte Amalie. Do not buy a round trip, you may have to take a different ferry when you go back to St. Thomas. Or we had a couple who purchased round trip, but couldn't find the tickets when it came time to leave.

You should have time to go upstairs to the Pump room and grab a drink and a bite to eat.

You can find a copy of the custom/immigration form here, if you would like to review it. http://asbvi.com/wp-content/uploads/2016/05/new-custom-form.pdf

While waiting in line be quite and polite and you should not have any problems.