I had an excellent one made by the folks at El Paso Saddlery for my MKII
5.5" Target which should also do great on the MKIV.
https://epsaddlery.com/product/2-thumbreak-2-to-5-12-barrels/
Mine is in the russet color, to me it looks better than the black that shows
every minor ding to disadvantage.
I have NO idea why the Ruger drop down model list doesn't include one
for a 5.5" Target auto but I got one somehow, lined and fits like the proverbial
glove.
In all fairness, Barranti does EXCELLENT work as well and I am hoping to
resume a dialogue with them on making me a custom shoulder holster for
a Ruger Slabside 6 7/8" model with a 2x Burris on it, we were right in the
midst of final details when I got rather sick suddenly so that I couldn't
get online to let him know; hopefully he has either forgotten about that
or forgiven me.