Anyway, my reasons for changing companies so "frequently" goes from the salary to a mismatch between job description and job assignment.
Another reason for having changed was that I spent 80% of my time doing nothing.
Are these not reasonable reasons to change.
To be blunt, those aren't great reasons to change. If you didn't like the salary you were offered you shouldn't have taken the job (unless you were desperate, but you said you weren't having trouble getting interviews), and if you weren't given enough work you should have found something to do. That's part of what people are reacting to when they call you a job hopper.
Another part of it is how you explain your reasons for leaving those jobs. If you take responsibility for having chosen poorly, that makes you look much better than blaming other companies for offering you jobs that you turned out not to like.
To be fair, salary can be a good reason to change jobs if your expenses rose for a reason you couldn't control and I've heard horror stories before about people getting penalized harshly for supposedly "making their bosses look bad" by trying to do more work or working in a place where job duties are very strictly regulated and they really aren't allowed to "do someone else's job."
That said you've got to be careful how you spin those. In a comment on another answer you mentioned that at the job with the low salary you expected to learn more in exchange for accepting the low salary, I would not mention the salary and instead say that you learned you would rather be judged on the results you get than on how many consecutive hours you can spend at your desk.
Third job: 14 months, Fourth job: 8 months, and I have been in this new job, that I want to change, since the beginning of the year
No matter how good your reasons for leaving each job are that kind of trend is going to make employers very worried you'll get bored and leave their company in a few months too. If you can possibly stand it I recommend staying in your current job for at least 1 year, ideally 2.
It looks to me like you're caught in a cycle of bad job -> desperately jump to another one -> another bad job -> desperately jump again. I think you need to break that cycle more than you need yet another bad job. If I were you I would focus on making friends with other people in your industry who can tell you honestly if you would be happy working at a company they've worked for and not change jobs again until you're really sure the next one will be a fit.
Or like another answerer said, go into freelancing. If you current job is dull, doing a little freelancing on the side (if and only if you're allowed under your current employment contract) could keep your boredom under control and you could eventually build up enough clients to drop your day job and freelance full time.