Well, you can't tell whether she'll pass the exam until she takes it, really. Seriously, every year there are surprises and so much of it comes down to how each child performs on the day of the test. So if your dd likes the school and is OK about taking the test, and you are all psychologically stable enough to go through the whole process (including the possibility of not passing the test), then your dd may as well give it a go. [That last comment was just a little light-hearted.]
Tutors - Journalists love writing those sorts of articles, take them with a pinch of salt. Yes, there will be the (possibly over-rated) popular ones, who will have been booked since reception. But there are many, many others.
There are many resources you can buy and advice you can access. Visit the school/s you're interested in. Make sure you know what the exam is like. Track down practice papers, make sure your dd has covered all the subjects she should have, at the right level. Make sure she's familiar with the form of the tests. You can do that. Or you can look for a tutor in yellow pages and make sure they do that.
Have a look at here. Hunt around on the website, there's a lot of good advice, including how to check out if your tutor is OK.
(Warning: This site is very helpful but there are some intense people on this site. It's not completely representative.)
Year 4 is not at all late to be starting - in fact, most people would say it's early. If you want to start tutoring, it's probably just advisable to check your dd's school has covered everything it should have, and in enough depth, and your dd understands what she's been taught.
And be calm. North London selective schools are competitive, so it's worth developing a chilled attitude, right from the off.