Doubling your 2 mile time will not give you an accurate 4 mile time. Fatigue is a curve, getting steeper as you go. Here is an average NCAA Division II trained college runner's times for a 5 mile race.
1 Mile 4:45
2 Mile 5:00
3 Mile 5:08
4 Mile 5:17
5 Mile 5:20
That averages out to 5:06 a mile but you can see that the first couple of miles are much faster, the third mile is right about on average and the last two miles are significantly slower. That's just the way fatigue works.
The above times are not mine, just typical of what my College Team Mates ran.
Incase some of you guys think this is fast, it's not. 5:06 pace for 5 miles is pretty average for a Division II runner.
I was a little worse than average but here are my times from College just to put it in perspective:
1 Mile 4:38
2 Mile 10:02
3 Mile 15:32
3.1 Mile 16:04 Track/15:30 Road
4 Mile Unknown
5 Mile 26:04
6.2 Mile 33:25 Track/32:30 Road
When I went to Selection, a lot of guys thought they were fast, running just under 7 minutes per mile for the eval. I was the fastest guy in the 4, 5, 6 mile evals in both of my classes. When I graduated, I went on Leave and had to run with the Girls Cross Country Team at my old College because I was too slow for the guys. And some of the girls still beat me and I was still running sub 6 minutes per mile.
Speed is relative. Have fun comparing yourselves to each other or to what I have posted. There are tons of people a lot faster than any of us. Then again, they don't have to pass CCOC or Indoc either. All that matters is PASSING every event of the eval. Running 4 minute miles won't do you any good if you fail push ups. Doing 35 pull ups, likewise does you no good if you fail the
water Confidence Eval. Worry more about passing each event than being great at any one event. I never had any problems with PT at Selection (I was always just 5 push ups ahead of the number each eval but everything else was easy) but WaterCon was tough. I was even set back because of failing a WaterCon event twice in a row. All the running and cals couldn't help me with that.