So I'm putting together my new fuel rails and the fittings are only going about halfway into the rails. They go about four full twists in and then just stop, but there's still plenty of thread left inside the rail. Is this right?

 

Doesnt look right. Thats gonna leak bad.

 

The fittings you are using are taper pipe threads, which will seal fine with four threads engaged (although 5 would be better). You don't use an oring to seal those threads, however you should use teflon tape to prevent fuel from passing through the tread root. Just don't get excess teflon tape in the fuel rails since it will plug up your injector screens.

 

Speaking from a guy that changes his fuel setup twice a year,since 2005, They will seal up just fine.
I used teflon tape and tightened the adapters on.
Yes,you see threads but that is normal..trust me I had a big chat with the Company over this.
That rubber grommet won't stop ****,so just don't use it...again,I had a aftermarket rail with rubber grommets.Popped the hood to see Fuel hitting the hood..above the turbo..scary stuff.
If you really want to "hide the threads" use a lot of copper crush washers.At least they do something.

 

Stock S4 rails with -6AN adapters will be 500hp friendly on pump gas. Just my .02¢

 

Awesome, guys. I appreciate it.