Does your family have humidifiers on the furnace? They might and just not realize it.
We heat with wood which is a very dry heat anyway, we use one of these large humidifiers and it works great for us. We could probably use a second one, but our home pretty large, 4800sqft.
High-end acoustic dealer here. So only speaking for acoustics.
40% isn't that low, but over a long time it could cause some minor issues. 30% is a different story. You want to aim for 45 to 50%.
Here's what I'd recommend for 4 guitars. If you have a room that can be sealed off, just get a whole-room humidifier. When it gets really dry/cold, you'll have to be able to close a door for the humidity level to rise. A really nice humidifier is like $100 to $150, and you just have to replace a filter once a year or so. Get one with a humidistat so it can shut off when it reaches a target humidity, and you're good. You'll only have to fill it every couple of days. I'd recommend using a bacteriostatic solution with every fill - it's like $10 for a bottle that will last you 2 years. I use this one:
Sooooo much easier than dealing with the in-case solutions.
This is the one we have, also because we have an upright piano: https://www.amazon.com/gp/product/B00I9YFXSS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It's big and can be loud...but it works.
I'm also pasting a comment I made elsewhere re: humidifiers: