As far as I can tell there is absolutely NO WAY to get upgrade levels higher than 3 using the standard starEdit. Luckily you're in the right place to get a tool that can help you. Go to the stickied thread at the top of the forums, read the whole thing, and get an extra editor or two. Good Luck.
This program has 5 different staredit modes. Classic, P12, Base, Alternative, and Special. Each mode has different purposes. Special mode can allow you to place units basically anywhere. However, none of the modes can allow you to stack units and other stuff. It has a built in color tool. It lets you have instant upgrade. Allows up to 255 upgrades. Basically the best staredit program out there.
Just a more advance version of Starcraft X-tra Editor 2.5. This version can be bad for you because the the trigger screen takes up the whole screen. You need to be under 1024 x 768 pixels to view the trigger screen.
SCXE or Sc X-tra Editor will automatically set the upgrades to 255. and For Instant Upgrade, press Control + Shift + U, to go to the upgrade window.
On the right side, it should tell you how to do Instant Upgrades, if not, just type "8739" in the Time Cost