400 psi in a plastic pen? That is not a good idea. Those pens are kind of brittle, so if one failed in your hand it would hurt a lot and also cause serious injury. Please use SCH 40 steel pipe for those pressures.

You know that pens are not rated for pressure, right? If that pen were to fail while you were holding it your hand would be very badly injured. If it's not pressure rated, do NOT use it.