I'm assuming you'll already have your 4th tab setup and ready to go.
I started off by making my 4th main menu tab a DVD drive launch button which is fairly straight forward, but I'll put the code snippet here anyways.
First search for this in your default.xap file
Under that heading locate:
This defines what the top/first tab on the main menu does when it's selected.
All you have to do is replace it with
if(nCurMainMenuItem == 0)
Then of course you must go to Settings/Dash Configuration/Main Menu Tabs and rename the 1st tab to whatever you want. For example, I used "DVD Drive Launch". I also went and disabled the "Autolaunch Inserted Media" setting in the Settings/Dash Configuration/General Configuration by setting it to "False". Since I didn't need it anymore and I dislike pop-up windows, but that's totally optional.
Now that you have a drive launch tab you have to modify the autolaunch function.
Replace with and edit the path to DVDx if different:
Your launch button should now start DVDx automatically.
Now on to the second part and where I'm having trouble. UIX would still launch the built in DVD player if a DVD was in the drive when UIX started so I had to do some more work.
First search for:
b_dvd = true;
I replaced it with:
Now UIX no longer autolaunches the DVD, but strangely there is no ambient audio (button sounds/strange background sounds) when UIX loads. Also, when you try to remove the DVD the Xbox reboots. If I start UIX without the DVD then put it in after it's started I can put it in and take it out with no reboot. So I'm stumped at the sound and reboot problem. I'm by no means an expert, and as you can see this is my first post so hopefully somebody has a solution because I'd really like to get this working.