Andrew, decent paces. I will try to forget that you're currently a XC star and saying how you found it hard, as I'm pretty much looking to do exactly that session and pace tomorrow...but as a 4mile continuous block
I reckon you could Dachs. I did one as part of a casual Sandhurst "fun" track night a few years back, 15mins after a 1k flat out rampage. I remember being gutted at doing 60 as a fellow Sandhurster,...not Graham, did 58.
It's a different game of running though. The feeling in the lungs is quite unsettling. No wonder they say you should start out short distance and build once exhausting your development at each distance...so much harder going back down.