You wouldn't consider a handgun with a capacity of four rounds as adequate........why is it OK with the shotgun? Why would you knowingly go into a deadly incident with 4 rounds?
while there is logic in this, there are also "counterbalancing" factors. One of them is the legal restrictions on firearms design.
The bedside/house gun is meant for use inside the house, more than anything else, so shorter is better. Capacity? sure, four rounds isn't a lot, but they are BIG rounds, and I doubt I'd be trying to stop the Terminator. (and my home defense shotgun is a winchester model 12, which has one more round capacity than the remingtons
)
For my wife, the choice is a coach gun. She's not a gun person. Double hammers, double triggers, 12ga, doesn't get much simpler than that, and if she ever needed to use it, simple is a good thing.
I would use a butt cuff or the sidesaddle (if it applied) for spare ammo, before I'd go with any aftermarket magazine tube extension, but then, I'm not very concerned with the possibility of an extended Hollywood type shoot out inside my home. But, that's just me.
Nothing wrong with using a slug gun for home defense, it would be handier than a 30" duck gun, certainly.
Assuming it works flawlessly, the only downside to a magazine extension is if it adversely affects the balance of the gun but more importantly, if you ever do need to use it, and face a jury, the prosecutor is likely to do their best to make you look like a foaming at the mouth monster because you made the gun "more deadly" and sadly, these days, that is something must consider.