This can actually be accomplished a few ways, depending on how complex you want to make it...
There is a Direct Trunk Access code that can be dialed, followed by the trunk number... This can be done manually or put into a speed call key.
You can program the line as a private line on the phone, press the key and dial the number.
You can use ARS COR and Route Lists to accomplish this, but it can get complicated. You need to give this user a different COR and make a new route for the single L4 trunk with only that user able to access it via COR, then all dial patterns would go to Route Lists, the first route would be the one with the single trunk, and second choice would be the existing route. User A dials 9 and a number and accesses the Route List and goes out on L4 as the user is allowed, User B dials 9 and the same number and the first Route in the list is checked but COR doesn't allow them access to it, so it tries the second route, which the user is allowed to access. It's a little more complex than I am saying to program, but it works well, especially if you have multiple phones that need access to that line.
The other option would be setup another ARS leading digit of say "8" and duplicate the relevant entries in ARS dialed digits to access a route with L4 in it, so users dial 8+number to access L4, and 9+number to access the "regular" lines.
There are probably other ways, but these are the ones that come to mind...