As for the side of the tunnel, which is also paved, yes, use it if you can! It leaves that little bit more room for the cars around you.
In short then: don't use the first concrete patch at Deep Forest's turn 4. I'll allow it in qualifying, to ensure that no one is dis-advantaged in qualifying, but in the races, you will be penalized for gaining time or positions in the concrete patch, or as a result of it. If you need to use it, to avoid a crash, then hold the outside line for the entire corner, to ensure that you don't block anyone else's racing line.