In addition to Andrej's answer, keep in mind that you sometimes have to use specific versions to upgrade from one version to another or have to do data migrations. Also check your used add-ons in the app store for compatibility. About all of this you can find information regarding these topics here:
I think you can only upgrade one major version at a time. Even if that is not true, i would still recommend doing it like that. Major versions ussually mean major changes which means new bugs. It is much easier to address bugs gradually as you upgrade from version to version. If you upgrade from 4.8.9 to 7.8 directly, there could potentially be so many problems that you wouldn't know what to start fixing first.