I've written two at once but never three or four. I, personally, would be too overwhelmed. But when I was writing two, I would work on one until I hit a wall, then write on the other one for awhile. I found it made it easier to get back to the first one when I had been away from it for a little while. And then I wasn't stressing about not making headway, because no matter what, I was making headway somewhere. I finished them both.
If you're starting all these novels but never typing the words "the end" then you are overextending yourself. The others are right, if something pops into your head, rather than starting yet another novel, jot down the ideas or even the first couple of pages, then get back to your original. It will quiet your characters, keep your ideas, and let you finish your first novel. The easy part is starting, the hard part is finishing but if you want someone else to read your writing, then you have to finish it.
Good luck!