I have a Colt 6920 that I'm thinking of sending the trigger in for a 4 lb mod.  Can someone tell me what the speed hammer mod is or if I even need it.
I primarily shoot reloads with the occasional surplus ammo.

Send an E-mail directly to Bill.
I'm not sure, but it sounds like he removes the metal on the back of the hammer to reduce weight.
I personally don't use lightweight or reduced weight hammer springs, so I don't need lightweight hammers. Which brings me back to E-mail Bill and tell him how you plan on using your rifle and go with his advice.

Bill has my 6920 trigger right now and I didn't opt for the speed trigger.

I think you'll get more consistent hammer drops without it, especially if you'll be using any hard-primer mil-type ammo.

I'd suggest you just get the basic trigger job and enjoy that, but ask Bill and decide for yourself.

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I had the hammer bobs on all six Bill mods.  Seem to work fine with factory ammo.  Highly recommended to get the trigger job.  The hammer bob is VERY optional, and if in doubt, don't.  Can't hurt NOT to do it.  The improvement with JUST the trigger cleanup is SUBSTANTIAL.

And, yes, he whacks off the tail of the hammer.  Less mass, faster speed, less latency form the time you pull the triggie thing.  Great for competition with known ammo.

I emailed Bill and he replied with this...."The speed hammer involves removing the hammer tail

for better primer ignition on military hard primer ammo, this runs $5."

So I take it as if he does the speed hammer, it is better for hard, military primers.