GP100 (or 141, your choice) is excellent as a house gun. But I prefere the 6" barrel. When the .357 was developed, 6" was the standard barrel length. 4" was a compromise to make holster carry more convenient (especially when seated in a squad car). Ballisticaly, the 6" is better and for target work gives a better sight radius, too, for tighter groups. 2" more weight out front also makes the gun balance a little better in my opinion, too.
Some people prefer the 4" because of the possibility that a 6" could be wrested away from you in a hand-to-hand struggle. My opinion is that 2" is not that much. Reduced muzzle blast, flash and better ballistics carries more weight in my decision-making process than that. It is up to you. The 4" is more balanced in my hand than the 6" when carrying and pointing. But the 6" is more balanced in my hand in the instant when a bullet is launched. You may feel differently.
The GPs do have enough weight to make even .357 loads comfortable to shoot and 38 specials downright easy.
You comment about finding a house shotgun being more difficult to shop for than a house handgun confuses me. Remington 18" 870 is the hands-down favorite. Mossberg 500 for the budget minded. Simple.
I recommend you learn to reload. Practice is one of the most vital elements of becoming proficient (accuracy and handling competence) with a handgun. If you don't have a companion/matching 22 rimfire, the next best thing is cheap ammo (which means, you rolling your own). But that is unsolicited advice.
Penetration. Yes, it is a concern. Choosing bullets that are less likely to penetrate your walls is one thing. Choosing tactics to reduce that likelihood is another. If you in-home tactics involves a stationary defense with a narrow field of fire (like down a stairway or a hallway) you can reinforce the wall/backstop at the end of that lane with a full bookshelf or a solid wood panel or such.
Thanks for asking our advice.