Imagine you have 4 running blogs and they all do okay. They are all on subjects you are familiar with and everything about your blogs reflect that. Would you consider starting a 5th one on a subject you know nothing about, and just hope for the best? Fake it until you make it?
The worst thing you can do these days is try to fake it online as an expert or authority on a subject that you know nothing about. There is just too much information out there that people will immediately see through you as a fraud.
You have to venture into something that you have experience on. There's nothing as bad as starting a blog on a topic that you know nothing about, then abandon it halfway. I'd rather concentrate on my niche and get the best out of it.