I'm not sure how the app comes up with the number of $4 dollars, but I have a couple hundred listings that have that as the default since I didn't bother to change it every listing, and I get a fair number of offers for my items that are below the cost to ship. Buyers must think shipping is really free, and I can somehow grind out a living selling things below the cost to ship them. I try to be polite and respond with what I'd take as a counter offer, but they are already anchored at their 5 dollar bid and it just goes no where. I will usually decline the next offer of $6 dollars. If they keep low balling, they get blocked.
So yeah, I think the minimum amount to ship something First Class in the US is over 4 dollars, this default amount is bogus when seller pays for shipping.
Consider a setting that allows you to set a percentage instead of a dollar amount. This way you don't have to change it when you lower your prices. It would automatically scale to your BIN price.
