are the updated newer major-versions significantly more secure than updated older ones?
This is quite ambiguous. How significant or insignificant security changes might be, it is totally irrelevant. There might or might not be significant security enhancements between minor and major versions or even major versions as such. To really know how significant changes are between versions, you will need to go through change logs, release archives and bug reports
Is 4.4.2 (from 2/2/2016) much more secure that 4.0.10 (also from 2/2/2016)
In all probability, yes. From what I could pick up, there where two security issues in WordPress v4.4.1 which where fixed in v4.4.2 and it was also fixed in v4.0 via v4.0.10. Apart from that, how v4.0 (through its minor versions) kept up with security updates in regards to v4.4, I don't know, this is something you need to backtrace. You need to remember, older versions are not actively maintained, so most updates might not reach older versions. A note from the release archive
None of these are safe to use, except the latest in the 4.4 series, which is actively maintained
And will 4.5 (when it's first released, as 4.5.0) be much more secure than 4.4.2? Or might it be less secure than 4.4.2, since 4.5 won't have had a security update yet?
This is impossible to answer correctly. There might be some minor security issues from v4.4.2 which will be fixed in v4.5, but to be sure, you will need to dig through the bug reports. In general, newer versions should be more secure.
I think you are really missing the bigger picture here. Apart from security updates and bug fixes, why do WordPress release major versions? The core developers made a promise to release two major versions per year, each one with at least one new feature which is totally open to ideas from anyone. Apart from that, a new theme gets released once a year which follows the year in which the theme is released. For example, this years theme is called twentysixteen
It is true that, with every new feature or major changes to current features, there will be bugs of some sort, which might or might not lead to security issues. There might still be hidden security issues from previous versions yet to be discovered
Keeping WordPrerss up-to-date has much more benefits to just security. Since the release of WordPress 4.0, all major releases after that saw significant changes with a lot of new exciting and very helpful features being introduced (and some not so great, like emojicons).
From a pure security point of view, it is definitely beneficial to stick to the latest stable release as it is the version that is actively being maintained, and therefor should be the most secure version. Any minor release, like version 4.0.10 is not actively maintained, and might have missed other minor security updates in the past.
As final thought, all of the above is totally irrelevant and a waste of time if your PHP version is as old as the dinosaurs and your theme and/or plugins you are using have loopholes in them the size of the union building. Remember, WordPress which uses PHP as language, is just a platform for themes and plugins. It really is of no use building a proper strong foundation (up-to-date WordPress) on a sandy beach (outdated PHP) and then building a house out of straw (bad security themes and plugins) and hoping that your house will withstand floods, tremors or fires (malicious attacks)