Converting a 9x19mm to a .40S&W....
Slow your roll....
If you want to convert a Glock 9x19mm to a .40, you'd need to replace the slide too(and maybe the springs).
A .40 caliber base may not cycle correctly with a 9mm(.38) type slide.
The dual recoil springs of a Gen4 may be different too.
I'm not a Glocker but you may want to check with a Glock trained armorer or gunsmith before you tinker around.
Speed's fine, but accuracy's final.