I've been doing the 2-finger test, but will change to the pinch test from now on.
Just my opinion, but 4 fingers sounds a little loose to me. My DS sometimes gets in a mood where anything at all is "TOOOOOOO TIGHT!", and I tap the adjust lever or do some other nothng thing, and he stops complaining.
I used to do it really really really tight, as in ONE FINGER, but then I moved to 2 to allow for some movement.
I've heard that "too tight" can actually cause serious injuries if the head is thrown forward and the body doesn't move a little with it. I am not sure, though.
Speaking for my own experiences, and what I've heard other techs say, we've never seen a harness that is over-tightened on a child. I'm not sure about "too tight" being able to cause injuries, but in all honesty - too tight just isn't going to happen... kiddo is going to scream if you get the harness so tight it's pressing into their skin.
In a rf'ing car seat, the risk for head and body like that really isn't there - that's the whole reason why infants and toddlers rear face and why it's safest to keep toddlers and preschoolers rf'ing to the limits of their seat.
In a ff'ing car seat, the head is going to come forward like that regardless, and I'm not sure that the shoulders being able to come slightly forward would really be a good thing - the thing you're accomplishing by having the harness properly tightened and the chest clip in the proper position is preventing your child from being ejected. Kids shoulders are very flexible, and with a harness that was slightly too loose, they could go straight between the straps and be partially ejected. That is the very real concern.
Most likely 2 fingers isn't loose enough to cause that problem unless you have a too low chest clip, harness strap wrong height or else really big fingers. Differing sizes of fingers are the reason why the pinch test is recommended now though, so switching to that will make sure the harness is tight enough. :thumbsup: