If you are really thin then you'll probably not want a horizontal for a 4".
Take a look at the Bianchi X-15 - Vertical
The secondary retaining strap is usually not needed at all.
One problem is that the spring steel retention makes re-holstering a bit of a PITA for practice drawing but, who cares...really.
It will allow you to carry vertically and have the firearm break out of the rig for a decently fast draw and presentation.
In an actual self-defense senario you grab the firearm and pull.
Not very complicated and in the aftermath of a self-defense shooting you'll not likely be re-holstering before the LEO arrive or after they arrive since the LEO will then take control of your firearm.
Galco International also has a vertical shoulder rig for your 4" Ruger.