One major complaint I've had with all the Marks is the "holster-ripper" front sight. Years ago, there was an aftermarket replacement called the "Sport-Site" but it is no longer made. Ruger apparently realized that some people were not happy with this sight, and now offers what it calls a "Standard Front Sight" in its web store. http://shopruger.com/Mark-IV-Mark-III-Standard-Front-Sight/productinfo/61018/
View Quote
Upon further investigation, it appears that this "Standard Front Sight" is too low for use with the Mark IV's adjustable rear sight. A better solution might be the ramp front sight from the Ruger Single Six (available as a part on ebay and elsewhere). It can be used as-is, or if you're a perfectionist, the bottom can be ground a bit to fit the Mark IV's bull barrel contour better. I used a Dremel grinding wheel of the same diameter as the barrel. If you're careful, the job looks absolutely professional. (If you're not careful, you could easily mess up the sight.)
With the Mark IV, the loaded chamber indicator and the internal key lock are gone. That leaves the magazine disconnector as the last "lawyer-mandated" feature. We will figure out a way to remove it.
View Quote
The Mark IV magazine disconnector wraps around the hammer on both sides (the hammer is inletted for it on both sides) instead of just on one side like in the Mark III. Therefore, if you remove the magazine disconnector, you have to replace the hammer either with a Mark II hammer, or with a Mark III hammer and a Tandemkross bushing. I imagine that eventually, Tandemkross will offer a double-sided bushing, so that the Mark IV hammer can be used. It looks like the Mark IV hammer has a large lightening hole, like the Volquartsen "speed" hammer.
The sear has also been redesigned. It looks like the old type sears can be used instead.
Clark steel triggers are still being produced, in batches. You can order one directly from Clark Custom Guns ($30, plus shipping, but well worth it). IMO, the Clark trigger is the best aftermarket trigger for the Mark series.