From my brief time looking over a MkIV...
The MkIV is better made on the whole over the MkIII. The MkIII is stamped and welded, and you can tell "price point" was important, so its not the cleanest job, especially the inside.
Takedown is a pain, even with some experience, its not fun. Though, my 22/45 (actually the fiance's) is easier to maintain.
The mag disconnect is easy and cheap to remove. You only need to buy and install a $10 bushing.
The LCI can be removed and it costs nothing, unless you just have to have the slot filled in.
I have no doubt that the older barrels are superior to current factory barrels. Seeing as the upper receiver is serialized, changing barrels is not as simple as swapping a part, so having a better factory barrel is good.
But this could be relative, as my MkIII Target (well, fiance's) has a very good barrel, I find it plenty accurate. Maybe a competition shooter with higher skill would find deficiency, but I have no complaints.
I have unfortunately had issues with Ruger's stainless barrels. All that I have had, hate lead bullets, and can only shoot copper washed. The barrels lead up quickly, and after only a few magazines there would be keyholing. I believe it is because the barrels have all had at least some chattering of the rifling cutting tool. Fortunately the problem would lessen over time, but I have not seen it go away entirely yet... may need more rounds through them.
If I was looking for a new in box Ruger Mk pistol right now... It would be a MkIV.
Not only for the easier takedown, but the controls are laid out better too.