It is hard earning any kind of money as a writer. Most people can read and write and it doesn't necessarily seem that hard but there's a bit more to it than that.
OP, if you've sold four articles you're clearly not doing too badly. If you can find a niche and build up some regular work you're doing well.
But generally, people are naive to think they can jump into freelance work and make money.
The best way to do it is to make yourself an expert in a particular field and then you might get decent freelance, regular work or even consultancy work.
re. trade mags, I work for one and we don't have any freelancers. Often people write for us for free.
I've done a tiny bit of freelance for other magazines but most of it has been based on my connections and has faded away now - there's not a lot of money out there to get in the first place.
Also journalistic writing and creative writing are different beasts. I'd love to sell a novel too (have finished a couple) but I'm realistic that doing so is a long shot. The first step is actually finishing and polishing a book - most never manage it. The second is writing another book instead of pinning all your hopes on dreams on just the one.
It winds me up when people say 'oh yeah, I want to write a book' and they've never even typed out the title. I'm not exactly the most disciplined writer either, but I'm here at my computer most nights, at least, trying to get better.
It's easy to talk about writing. It's hard to finish things.