[930gt-40r]

Pete, I am sorry to say....
I am pretty sure you shifted the 3/4 fork on the shaft internally. I say this because I have broken my gearbox a few different ways now. One way is if you bent the fulcrum fork that bolts into the bottom of the trans, but this would make a weird, constant grinding noise while in gear. Then there is shifting the 3/4 fork- it will go into 4th just fine because the fork is now slightly closer to the 4th gear synchro inside the trans but also further away from 3rd.
I hope it is something external for you, but performing many "vin deisel" shifts and breaking things myself, I feel it may be a little more towards internal.