It took me a while to finally getting around to playing with mm4 but once I did my enjoyment of it started as soon as I opened it up, I enjoy it so much I ceased using MM3 30 minutes after upgrading and converting all my scripts over. So as I did with the upgrade to MM3, here are my list of things which I enjoy the most with MM4.
--Tabs - which are remembered between MM sessions. The only thing that this lacks for me is the ability to move the tabs around.
--Media Server - which allows me to use my Netgear to play my music/videos and have the play counts updated in MM. The only thing which is missing here is the ability to access the server from outside of the network.
--Media Tree - The ability to add specific collections to the media tree is very helpful because I browse my collection differently for media types and when editing tags. Also, the ability to have specific columns visible per collection is great. The media tree also now has the ability to allow you to re-organize the order the nodes appear, a plus in my book.
--Video Syncing - with MM4 I am now able to sync my videos and music to my zen with one application, this was something that I did not think I would want but I am enjoying having this feature.
--WDM Accessible from Media Tree - Now with MM4 I am able to browse my zen player, an ability that has never been available in MM. It is nice to be able to scan my player while in MM so that I can see if I need to upload tracks to my player. In MM3 I only had the ability to access playlists, now I can access all the folders as well.
--Custom Toolbar - Now with the custom toolbars I am able to save toolbars on the now playing bar, something that was not available in MM3. I use the custom toolbars alot and the NP bar more than any other area so this is a big bonus for me.
--Video Playback - the video playback in MM4 is crisp and clean with very little stuttering, and this on a machine that does alot of multi-tasking.
--Auto Conversion -- I do not know if this is new to MM4 but it is new to me because this was something I never needed to do, but with MM4 I can auto-convert my files quickly before sending them to my streaming player, this is a major bonus for me because my player needs the videos in a format that I currently do not have them stored in.
--Script Migration - Although this is not directly MM4 related, I love that I was able to migrate my favorite MM3 scripts to MM4 with little loss of script or functionality. There are a few that need tweaks but my major scripts are still accessible. This was a deal breaker for me because I am very heavily dependent on scripts. But I only lost one major script but thanks to magic nodes I was able to convert my nodes over to MN. The only other major script which I lost was able to be replaced partially with the tabs, so all is good in my world.
I am sure there are alot of features that I am missing but these are the ones that I have used the most since migrating to MM4. And now off to find more treats... Please share your thoughts on your MM4 experiences because I would love to explore it more.

Nyn