4-Layer PCB USB Routing Question
For FS mode of operation you don't need to keep precise impedance of traces. The trace impedance control is only important for HS modes (480 Mbps) and above.

It is also strange that you didn't mention the "stack-up" of your particular PCB design, which is critically important when selecting trace width for impedance-controlled design.

Just for reference: ExpressPCB (quick turnaround for 4-layer PCB) offers 0.305 mm dielectric spacing between inner (ground) layer and signal layers. If you want a 50-Ohm trace, the trace width must be around 0.51 mm wide. There are plenty of PCB trace impedance calculators that allow pretty accurate trace selection, Google for "pcb trace impedance calculator".