The tag on GitHub at v0.1.1 tells me that Jekyll 3 is only supported
But the latest code on master that is not tagged and on RubyGems will actually support Jekyll 4
So… if you are feeling adventurous, you can install the absolute latest code from manager plugin on GitHub as per these instructions of mine.
There is the risk that the code will break one day when you install, as you are choosing an unstable master branch over a tested and published tag.
But you could maybe specify the commit hash that is currently where master is and get the gem command to have that commit hash as a reference.