4.1 SP1 is not mentioned in the article because we support the latest 2 releases - SP2 & SP3 and the official 4.1 release.
If you have a project that is not using Sitefinity 4.1 and you want to upgrade to 4.2, you can upgrade it to 4.1 1339, SP2 1501 or SP3 1574 and then to 4.2 1650.
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