I checked the RPM NTC3 bumper the other day and tried it on the spare TC4 bumper I bought. I am still waiting on the car to show up in the mail, so I haven't built and tried this out. But I think it won't fit properly. The middle large hole in the back on the RPM bumper specifically the back part will hit the raised part of the front bumper of the TC4. It looked like you have to shave off some material on the bumper middle hole area or the TC4 raised part of the bumper to allow the RPM bumper to fit.
Are you putting the grommet underneath?