There are many kinds of data on your developer preview disk:
S1. /System and the BSD subsystem (/bin, /sbin, /usr, /var, /etc, and more)
S2. /Library and many of its subfolders
U1. The Library folder in your home directory and other libraries, such as the iTunes library in the Music folder
U2. Third party application data in /Library and /Applications
U3. Your documents, images, etc. in your home directory in {Music|Documents|Pictures}
Even though they are in your home directory, the Library subfolder and the library of iTunes will be partially affected by the upgrade (in particular, ~/Library/Preferences, ~/Library/Mail). Those will be upgraded automatically when you first launch the corresponding applications. Since you have already upgraded to Lion, your Library folder is now in "DP4" state and SL is no longer able to make use of it.
Items in category U3 are not affected by the upgrade.
Items in category S1 and S2 will be ignored for the most part by the upgrade, save for some preferences such as your network settings, time zone, etc.
What follows is my current plan of how to upgrade from DP4 to final Lion, based on my experience with previous upgrades from DPs to SL GM.
First of all, make sure to have a second admin user on your developer preview disk (call it something harder to guess than "admin" though), and make sure it has as strong a password as your main user.
Keep a full clone (done with Disk Utility, CCC or Super Duper) of your Snow Leopard disk around for the next months.
When the final version of Lion comes out, clone your existing developer preview disk and keep that somewhere save as well. You will need it for the upgrade though. For total peace of mind, I will create two copies and only use one for the upgrade (see below).
Make sure both your cloned disks allow you to boot them up and all applications work fine.
At this point, your latest data is in the clone of your developer preview disk, so you want to migrate this.
So you clean install SL to your main disk, wiping it in the process (boot from DVD, choose Disk Utility from the Tools menu, format as Journaled HFS+, etc.). Don't install anything you don't need, it's a waste of time.
Boot your fresh SL, create a third admin user named differently than your other user accounts, login and upgrade SL to the latest version (likely 10.6.8).
Download the upgrade to Lion through the App Store. Now, before you start the upgrade, hook up your external disk with your Lion DP clone.
Once Lion is installed and done booting, log in as the admin user you created under SL and see if everything works.
Do any updates if possible. Give it a reboot or two.
Now open Migration Assistant to migrate your admin user you created at the beginning.
Log in and check if it works alright.
This serves the purpose of not wasting hours migrating your main user account with hundreds of gigabytes of data, only to find out it did not work.
Finally, open Migration Assistant again and migrate your main user. This is where you bring in the data from DP4 and people will say it's a mess. You do this at your own risk, I will try and expect it will work just fine. If it does not, you can always restore your SL clone to the main disk, upgrade to Lion and restore your data of category U3 manually.
Fingers crossed and good luck!