Cocaine is normally regarded as the kiss of death.
I'm sorry to tell you this.

As the folks above have indicated, these are generally disqualifiers. Even if it was one-time experimental use, it's likely to be discovered during the background check (esp. if you were as open about it to those around you as you are here). You should be honest about it and tell the agencies what the deal is - perhaps they'll make an exception. 

Anyway, let's take a look at, say, the FBI policy:

# Have you used any other illegal drug (including anabolic steroids after February 27, 1991) at all in the past 10 years?
# Have you used any other illegal drug (including anabolic steroids after February 27, 1991) more than a total of five times in your life?

You said this stuff occurred 6 years ago and you've been totally clean. If it was only those four occasions (and you can identify them as such - only a handful of times) - within four years, you'd be eligible to work at the FBI. Look into the drug policy of the department to which you are applying. 

I don't advocate this - I think being honest is essential to success in this process - but you can always simply try your luck. For better or worse, I know of at least two individuals who, despite some "experimental" drug usage that pushed them to the very fringes of the drug policy, have passed lie detectors and gone on to become law enforcement officers. Make of that what you will.

Where are you Applying?

NYPD will take you without a problem at all.

I have a friend on the job who used coke and some other things before he was on the job also, 

Then he did 4 years of good service in the Army, just like you, he got in without a bump in the road.