It's a shame you cant print out the tickets online like you can from Disney tickets site.
As far as getting in at 4pm. You shouldn't have a problem unless the park is crowded that day (it usually isn't). The answer I get each year is, "we will let the party goers in starting around 4PM is crowd levels permit."
Most people avoid MK that day, because they get charged for a full day and get kicked out early.
The earliest we have arrived is around 4PM and have always gotten in. We now go about 5 to 5:30 to have a early dinner. (Don't waste your party having dinner in the park).
Also, skip the first parade and RIDE RIDE RIDE there will be no wait. Second parade will have better views as most people went to the first parade.
We have had our best views from the bridge in front of splash mountain (get there 20 min before the parade to stake out a spot) Also, good views from around Liberty Tree Tavern.
When it comes to the fireworks, make sure you are south of the castle for the best views. I like the rose garden or the bridge to tomrrowland.
Anyway, HAVE FUN this is my favorite day at MK.
