Some of the best historical research I've ever seen was done by a landowner who wasn't a surveyor, and apparently was OCD about obtaining and providing any and every scrap of information which might prove useful to a competent surveyor. To me, KgatesKY, you appear to be as advertised, a well informed and curious landowner.
Your best advice so far is probably from eddycreek, since he's licensed in KY. If I recall correctly, linebender is licensed in CA, as am I. Here, as in many jurisdictions, it used to be that existence of a fence being used as the true boundary by the landowners over an extended period of time was considered to be very good, if not prima facie evidence of an agreed boundary location. Here in CA, the courts now tend to scrutinize fences more than they once did. Kentucky cases may vary somewhat on that score.
Generally, you need to look first for evidence that the fence may be evidence of the original boundary. As eddycreek said, first, look for the trees and stones called for in the older deeds along the fence line and it's northerly and southerly extensions. If one or more of those trees and/or stones can be found, either along the fence, along the 2008 line, or indicating some other line in the vicinity, they will prove the original line better than any other information or evidence you can find.
Whether or not you find any trees or stones that appear to be those called for in the old deeds, in light of the 1951 aerial, you might try looking up any old timers who lived on or near any of these parcels about that time and see if they can tell you anything about the property lines and/or fences. You need to be careful how you ask these questions so that you are not introducing a bias that the fences and property lines are or are not in the same place, but so as to let their memory of the facts lead to the conclusion. Easier said than done. The history of use and recognition is going to be your second best evidence and may serve to support a conclusion that the fence marks the location of the originally established true boundary location, or in the alternative, that it was placed by the affected landowners in the mutual belief that they were marking the true boundary to the best of their ability after the original monuments (trees & stones) were lost - that would be an agreed boundary.
The 2008 surveyor shows having found a couple of iron pipe survey monuments, set by LS#3455 at your SW corner and the witness point to your SE corner, and by LS#2474, 297' south of your SW corner, and based the location of the westerly line on those. The copy of the map you posted cuts off the north end, so we can't see what he might have found at the north end.
That 3 surveyors have already identified the westerly line in the same location doesn't necessarily make them all right, but it does raise the bar some on disproving the location they've all identified in favor of the fence location.
You said that you work with computer graphics. That being the case, you should be able to obtain copies of the most recent aerial image and the 1951 image, overlay one on the other, getting them to the same approximate scale using landmarks clearly shown in both, and from that determine if the 1951 fence is likely the same one that is 15' +/- west of the line monumented in 2008.
If, in checking these things, you find enough evidence that appears to support the fence location as the location of the originally established boundary, shop around for a surveyor who 1) can explain a thoughtful process for investigating and analyzing whether an existing fence represents an original boundary location (if they start explaining how it's more important to follow distances and/or directions, patiently hear them out, thank them for their time and keep looking), 2) have some working familiarity with appellate level court decisions in KY where fence evidence played a significant role in the outcome, 3) are familiar with survey and title history in the area, and 4) can explain a bit how they are able to determine whether a stone or tree is one called for in a deed or just another tree or rock.
Hire that surveyor to assess what you've found and to look for additional evidence (if appropriate) and render an opinion. It's important that when you go out and look for some of the physical evidence that might be there that you don't disturb it. For example, if you find a stone that you think might be one called for in the old deeds, it's half buried when you find it, but you dig it up to look for any marks, then when the surveyor gets there, he can't say with full certainty and integrity that he found it undisturbed in that position, which diminishes its value as reliable evidence. That is, if you have to get into legal action over the boundary, if the surveyor can't assert that he found it undisturbed, the other side's attorney will either argue that you moved it to your advantage before the surveyor got there, or strongly imply that you did since the position benefits you.
In short, if you find anything that you think might be an original tree or stone, don't disturb or damage it in any way, tie a piece of flagging on the fence or a nearby tree branch, and hire a licensed professional to further asses it so as to maintain the credibility of anything that turns out to be reliable indication of the original line.
If you don't find anything that you think appears to be physical evidence of the original line but believe you have good information that the fence may have represented an agreed boundary line, the first professional you should seek out is an attorney who includes boundary matters as a significant portion of their practice (as in looking for the right surveyor, look for an attorney that gives you confidence that they have a very good understanding of this area of law).
The fence between Parcels 4 & 5 appears to have a significant bend near the halfway point, with the easterly end being near the dividing line identified by the 2008 surveyor. I was having a very difficult time discerning whether a fence existed in that location in 1951, but see that there was a farm road across the property to the west that appears to end at the westerly line of the parcel your parcels were divided from. Following aerial images forward through the years, it appears to me that the westerly end of the now existing fence seems to roughly line up with the easterly end of that old road. If the owners of the northerly portion of the parent parcel of your property used that farm road for any reason, it would be a pretty good indication that the E-W fence there was placed for convenience of accessing that road rather than as an attempt to mark the true dividing line between what were identified as parcels 4 & 5 in 2008. When investigating this, again, form your questions carefully.
The Amish folks that live on Parcel 4 now may be the nicest and most honest people you can think of, but mere confidence that others have the character to do the right thing and the memory to remember what the right thing is several years down the road isn't of much value if you find yourself disappointed at some point in the future on either count. It's best to have an agreement in writing granting them permission to leave the fence in place until it needs to be replaced, at which time it is to be placed according to the boundary as identified by survey.
I hope that between all of us, we were able to provide you with useful info that gives you clear points to consider and from which to make a good plan of action.