A friend has a 686+ that drags after a couple of cylinders . He swapped from lead reloads to plated bullets and claims similar problems .

After a through clean he claims the cylinder gap measures only .004 . That's is pretty tight ?

S&W use to keep the cylinder gap at .005-008", with endshake under .001". Lately the acceptable gap is anything up to .012".

.004" is definitely on the tighter side, .006" is considered the 'best' by many.

My Dan Wesson Model 15-2 came with a .006 feeler gauge. That's obviously what they recommend.

Is the timimg out ? Shaving a bit ? If the timing is good then maybe open the throat slightly. Or..Knock off a thousandh then try.
and... Cylinder dragging in front or rear ? Lead\powder (DARK) scrape on front or heavy brass scrape on frame ?
What kind of loads ? Hot or normal ?

My 586 would start dragging after a couple/three cylinders of ammo when I got it back in 1981.  I read (somewhere) that the recommended gap was 0.004 to 0.006" at the time.  I got out the old set of feeler gauges from my tool box and my 586 was sitting 0.0018 or so.

Being me, I used a file followed by a stone to open it up to almost 0.004" and tried it out.  To this day it works fine and I've only shot one revolver more accurate than it.

I can't imagine 0.004 causing problems when mine is slightly less and works fine.  Not from a build up of powder residue anyway.

Is there a bunch of crap between the ejector and the cylinder?? Has the ejector rod loosened up??

A .004 gap shouldn't be a problem . I wish all my Smiths had that . My 629 is at .0025 and I can easily shoot 50 rounds of jacketed bullets before there's any sign of drag .
On my Freedom Arms 44 mag and .357 I can't even get a .0015 feeler gage to fit and they won't drag even after 100 rounds .

 
   ^^^  This  ^^^

Filing and stoning the barrel is not how you open barrel cylinder gaps UNTIL you know what the crane surface has done and know what the headspace measurements on the other end are AND make sure there is absolutely nothing under the ejector star.

AND made sure no high primers are forcing the cylinder forward.