"Just so we are clear", I know that the grenade fuses are chemicals that burn "slowly" and that it isn’t a atomic clock, I guess the manufactures of the m67 tossed a 100 hand grenades (or just lighted the fuses without any explosives

) and calculated the average fuse time to get that 4.2 seconds.
Also, the ability to cook the grenade in your hand wait 2 sec toss it and it would be an airburst would cause what I would like to call "Team fortress syndrome" and we wouldn’t want that.
Back on the topic of fuse times, It would be nice with a random fuse time if its possible since this would mimic reality as much as possible but its probably hardcode (what isn't?

). But at least we could have a realistic time that hopefully would cut down the nade spam or at least reduce its effectiveness.