Not sure what is happening for you on your end. Can you try deleting the block and then re-add it and using the setting in the block sidebar to set the header size. This doesn't occur for me on a fresh install see my screenshot.
Please try this and let me know if it helps.
Thanks,
Shane
It happens with any Heading Block I use anywhere on the local site and only happens when set to H4. I can also verify that it is only on this one site and not others running Toolset Blocks. it seems to be a compatibility issue with Editorkit. https://wordpress.org/plugins/block-options/. I did the plugin shutdown and isolated it to only happening when this plugin is active. I'm not sure how else to help sort this out. Can you install the plugin and see if it happens to your fresh install?