Perhaps I just don't understand something, but I'm still not seeing anything that would imply any restrictions on the type of government a state chooses to implement. To me, the additional clause you showed just says that the federal government shall protect the states against invasions.
> The Republican Guarantee is a commitment of the Federal Government to the States about the States.
I'm not quite sure where the "about the states" part came from. To me, Article IV just reads like a set of promises the federal government must make to the states.
The key word in my comment is "additional", which was intended to indicate that I was replying to "and shall protect each of them against Invasion".
I have to admit it's also partially my fault since I didn't notice at the time that you are not the same person who made the original comment I replied to, so I left out some context.
Otherwise I agree that it is just a list of commitments.
> The Republican Guarantee is a commitment of the Federal Government to the States about the States.
I'm not quite sure where the "about the states" part came from. To me, Article IV just reads like a set of promises the federal government must make to the states.