Exactly the same here. I waited for one hour, nothing happened, but the Quit button was still dimmed in the Install XCode app.
Thinking this might be caused by not uninstalling Xcode 3 beforehand, and noticing there was a /Applications/Install Xcode.app, I ran the uninstall scripts for both Xcode 4 in /Developer and Xcode 3 in /Developer-old, then force-quit the Install Xcode app which was still running and restarted my Mac. After restarting, /Applications/Install Xcode.app is still there so the installation can be launched again.
Alas, it persists in halting forever just before the end. Inspecting /private/var/log/install.log in Console, it seems the installation is actually finished but the install app fails to notice it. Here's the end of install.log:
Mar 10 11:18:21 [...] installer[257]: PackageKit: Registered bundle
file://localhost/usr/share/git-gui/lib/Git%20Gui.app/
Mar 10 11:18:23 [...] installer[257]: Installed "Xcode and iOS SDK (Partie 2 sur 2)" ()
Mar 10 11:18:23 [...] installer[257]: PackageKit: ----- End install -----
Mar 10 11:18:24 [...] installer[257]: Running install actions
Mar 10 11:18:24 [...] installer[257]: Removing temporary directory "/var/folders/[...]"
Mar 10 11:18:24 [...] installer[257]: Finalize disk "Macintosh HD"
Mar 10 11:18:24 [...] installer[257]: Notifying system of updated components
Mar 10 11:18:25 [...] installer[257]:
Mar 10 11:18:25 [...] installer[257]:
** Summary Information
**
Mar 10 11:18:25 [...] installer[257]: Operation Elapsed time
Mar 10 11:18:25 [...] installer[257]: -----------------------------
Mar 10 11:18:25 [...] installer[257]: disk 0.01 seconds
Mar 10 11:18:25 [...] installer[257]: script 0.00 seconds
Mar 10 11:18:25 [...] installer[257]: zero 0.01 seconds
Mar 10 11:18:25 [...] installer[257]: install 394.25 seconds
Mar 10 11:18:25 [...] installer[257]: -total- 394.28 seconds
Mar 10 11:18:25 [...] installer[257]:
Thinking of erasing everything and reinstalling XCode 3.2.6...
Plus, the App Store installation process does not seem to offer the option of installing the 10.5 SDK (and the 10.4 SDK) in addition to the 10.6 SDK. I hope they can be added afterwards.