I do like some of what you are saying. But there is way more to this then parallel and same length drag link/track rod relationship. The steering linkage really needs to be at a different spot on the axle/knuckle to make it work correctly on a 4-link. When your axle moves through the suspension cycle it also moves forward, up and over. Where the drag link connects on the axle works great with a radius arm setup. You install a 4-link on there and your axle doesn't go through the same exact motion since the caster hardly changes and that exact point where the drag link connects to the axle is now at a different spot then where it would be on a radius arm setup. It is a slightly different path in the 3D world. When you actually draw this in 3D software you will notice the difference.
Well I'm glad you like a little. And yes there is slightly more to the equation of bump steer but I'm keeping things simple and pointing out the direct main cause of "bump steer". By saying parallel drag link and track bar sets the foundation(for the less knowledgable) for solving bump steer. As for your point about caster angle changing less on a four link, which in turn puts the draglink/knuckle pivot in a slightly different location due to the lack of axle rotation, you are correct. What your saying is the draglink/knuckle pivot is farther from the axle caster center of rotation than the track/axle pivot. So when the axle roatates the draglink/knuckle location moves a further distance than the track/axle pivot. The arch path gets larger(dis-proportionally to the track bar)the more the axle rotates. That fact all on its own makes it very difficult to sync the track bar and draglink motion on a three link. A 5 link (if you want to get technical because a true 4link is a whole other ball game) eliminates this added geomerty calculation by eliminating caster change. If the distance between the pitman arm pivot in front of the frame/track bar pivot is roughly the same as the distance from draglink/knuckle pivot in front of the axle/track pivot, along with parallel/similar length track bar and draglink (on a five link setup), the archs will be virtually the same with no bump steer.
On a side note, were talking moderately lifted/no lift/ lowered superdutys. So the suspension travel/angles are pretty minimal to any average superduty drivers out there. The whole bump steer due to caster fluctuation is hardly an issue because their isn't enough suspension travel to feel small geometry flaws at the end of the day. Hence why their are happy guys with properly setup 3 and 5 link setups under the same ol superduties.
One other thing that sucks about a 4-link is the lack of caster change. What do you think happens to a front driveline when your front axle droops out. It needs to stay straight out of the axle and only change angle at the transfer case. Then when you droop out your front suspension the driveline will just blow up at the axle end and take out your oil pan, your transfer case and beat the piss out of the bottom of your truck until it falls off. So if you are building your superduty for going offroad you really need to address the front driveline if you run a 4-link.
The ujoint angle is not an issue on moderately lifted (8") to lowered superduties. If we were talking about +10-12" lifts then yeah we might be concerned. That's very far from the majority. The drivline does NOT need to be addressed for the majority, even if they off road. My 7" front lift/4 link bars set with a degree of extra caster above spec(which further hurts my driveline argument) goes to full droop without binding. If i pull the shocks, brake lines, sway bar i can push the axle down another 6-8 inches before the yokes start to kiss. The driveline dilema is a non issue.
Look at POS dodges. They went to a 4-link setup and now what do the trucks come with....it is very similar to a Ford now. Dodge couldn't even get it correct.
I could be wrong but I'm pretty certain ford uses a 3 link setup for simplicity, wear, and cost. It has two pivot wear points plus track bar on a 3 link vs eight pivot wear points plus track bar on a 5 link
This is my truck. My track rod is like half the length of my drag link. There is a lot more to the equation then just the drag link/track rod relationship.
Like I said, trying to keep things simple and touch on the main points for the majority.