I have seen it with certainty several times in recent years with a 110mm ED refractor in Bortle 2 to 3 conditions. Pluto is over half a magnitude brighter than my measured telescopic limiting magnitude with that scope, but it is also low in the sky which increases the difficulty. I wouldn't have any reservations about trying it with a 4" apo in similarly dark & steady sky. I have tried with an ED 80 in Bortle 1 conditions, but not succeeded--seeing was not steady enough to go deep that low in the sky, and I was falling at least 0.3 mag short of what I have done with that scope in the past. I have also seen it with my 20" stopped down to 6", but in Bortle 1 to 2 conditions.
Most of this sort of threshold detection comes down to learning how to see stars at the limit of your aperture. If you can detect stars moderately low in the sky down to 14.5 mag, you will be able to see Pluto since it is perhaps a tenth or two brighter than that. For telescopic limiting magnitude the final 0.5 magnitude is very difficult/tenuous and is seen as occasional glints in the same location when you direct your gaze so that the most sensitive portions of your retina happen to catch them while in focus. Once fully adapted at the eyepiece for the magnification/exit pupil employed, observing the same area over several minutes can allow one to sort the "noise" from the real detections.
You need a good finder chart down to 14.5 mag, and it doesn't hurt to use something like Wikisky later to check for stars somewhat dimmer than this, as you will likely see a few of them, and some can be near the same track/position or befuddle your pattern recognition.
A few years ago when my son was a complete novice with his 10" Dob I handed him the S&T chart for Pluto, explained how to employ it, and where to point his finder to start the hop sequence. I turned to tougher DSO targets with my scope thinking that would keep him occupied for awhile. He had it in about 15 minutes, but this was in Bortle 1 conditions. I checked in his scope and he definitely had it; I had no trouble verifying the field and Pluto.
Edited by Redbetter, 04 January 2021 - 04:29 AM.