NO. You can enter the park at any time during the day and stay for the party. If you use a regular ticket to enter the park early, you must show you party ticket at the Rose garden by the hub and Tomorrowland's entrance after 4pm and receive the wristband that is required of those attending the party. Without a wristband, a CM will ask you to head for the exit and leave the park after 7pm. I think those arriving at the Mk after 4pm will be allowed to use their party tickets to enter and will recieve their wristband at the turnstyles as they enter the park.
Plan to. I've got 4.5 days there, but that includes Islands of Adventure, Universal Studios and Halloween Horror Nights. But I just came from WDW two years ago, so don't feel the need to do everything...just the favorites and the new stuff.
I think you can actually go to town hall and get the wristband prior to 4pm if you want to, or at least you used to be able to do that a few years back... Saves you from getting caught up in the madness on main street of everyone entering and exiting the park...
I was not awared that you could do that at City Hall which is convenient if you happen to be at the front of the park, but I still believe it's more convenient to just go to the rose garden and get it done without waiting for other guests with other businesses at the City Hall. Took me only 30 secs to get my wristband at the rose garden... from 4pm on.