Once cracking the throttle above idle speeds it does indeed change the vacuum balance and one can get the airflow(vacuum) balance improved quite a bit. In my experience 2 or 3 of cylinders have been fairly close initially and the 4th has been off by at least 10-15 % quite often. During this step it requires tweaking the throttle blade balance to get an even vacuum at 4k or your choice of cruise RPM. It has generally taken me 3 or 4 attempts to get both speeds to be balances at the same time.
most engines at that speed are well into the ' torque curve '
at peak torque usually at ' full advance ignition '
resulting in the maximum ignition [ bang !! ] inside the cylinder
[ many mistake the event to occur at reedline ....]
4000 rpm has absolutely nothing to do with the idle
no less it has nothing to do with redline performance
so tuning at idle does NOT effect either
tuning specifically at 4000 rpm has NO effect at idle or redline
no less tuning WOT [ wide open throttle ] has NO effect at idle or 4k rpm
so im at a lost to comprehend what one might accomplish
the actual compression of each cylinder ..............SELDOM SAME ALL 4 cylinders
will be a greater factor than adjusting idle screws
[ a direct result of the ghost cylinder ................1 that fires directly after & beside
the prev cylinder at a distinct / evident disadvantage
no less harmonics in the inlet & exhaust tract present other quantifiable negatives ]
no less a accurate / blueprinted valve adjustment
will be a larger factor than adjusting idle screws
while 16 valves might be within operational tolerance
they are SELDOM accurately / exactly the SAME !!!!!
both compression + valves will attribute more to ' vibrations '
than the idle screws can EVER mitigate
no less over the overall life of an engine
while it in its CORRECT operational dynamics
you will NEVER have enough adjustment at the idle screws
to compensate for the ongoing realistic operational parametres
of the engine