FWIW, my 2 cents...
When I get a bike I have not dealt with before, one of the first things I check is for proper chain tension. To do this, I support the bike by the frame or footpegs and remove the shock. I then move the swingarm thru it's arc making not of the chain tension. The chain is at it's tightest point whent the C/S sprocket center/swingarm pivot center/rear axle center are all in line. The chain should have little or no play at this point. I block up the rear wheel at this point and adjust the chain. Now I install the shock.
It just so happens that with 99% of the bikes checked...road/MX/offroad...if you take your finger and push the chain up to meet the rear most point of the plastic chain slider on the bottom of the swingarm, the chain above the swingarm will be snug. Your chain is then adjusted properly.
The 3 finger rule does not apply...my 3 fingers equal your 4 fingers.
JMO Tdub