Keep in mind that the 4 oz listed for the FF socks is also split between 2 socks, so that is only 2 oz of down per sock, not 4 oz per sock.
Also keep in mind, the GFG socks are actually customizable, which means you can add as much, or as little, extra down as you want in them. You can't do this with the FF booties (as far as I know).
And yes, I own a pair of GFG down socks in blue M50 with 25% overfill, and I wouldn't trade them for anything!
Edit to add:
Thinking out loud...
I wonder about the actual down fill in the FF booties...
On the GFG site, for the size medium booties using 20D, Ben lists the total weight for the pair at 2.2 oz, with only 1 oz of that weight as being actual downfill. This means, a pair of his medium sized booties have 1.2 oz of other materials (shell material, thread & shock cord) per pair. (And IIRC, Ben uses the M90 from Thruhiker as his stock material, which has a finished weight of 1.1 oz/sqyd.)
If FF is using the Pertex Endurance LT material, (which is also 20D and listed at 1.22 oz/sqyd on the FF site), shouldn't the total materials weight be slightly heavier? Also, as Ken pointed out above, the FF inners even have snaps around the ankles (so 4 snaps total?) which may even add a little more weight.
Ken listed that his FF inners in a size medium weighs 4.79 oz. If the materials (shell, thread, shock cord & snaps) alone make up the same 1.2 oz as the GFG booties, then this would mean there is at most, only 3.5 oz of down fill in his pair of FF inner booties. However, if the total materials weight in the FF are indeed slightly more than the total materials weight of the GFG booties (which is possible due to the added snaps, and especially if they are using the Pertex Endurance LT material), then the downfill would be even less. Maybe closer to 3.2 - 3.3 oz, which is about a 18% - 20% less amount of fill that is listed.
OTOH, if FF is using their Pertex Endurance UL material, the total materials weight will likely be slightly less than the 1.2 oz total material weight of the GFG. So, this would likely put the FF total material weight around 1 oz. However, this would still mean that there is only 3.79 oz of down fill in his booties... which is a bit more acceptable, but still, slightly less.
I know... nitpicking... but still food for thought...