The Article IV Sec. 4 guarantee cannot be interpret as even coherent if a state can secede; once a state is admitted to the union, he federal government is irrevocably obligated to preserve it as a subject and republican government; if a state government could escape this oversight by secession, the guarantee would be empty.
Further, there is ample historical evidence that the idea of reserving the right to secede when ratifying was raised by New York, and rejected because it was understood that it would be viewed by the Congress as an inconsistent condition attached to ratification and thereby nullify the ratificstion.
The Supreme Court has also ruled on the issue, in Texas v. White. So your concept of a right to secession is inconsistent with the text, historical evidence of intent and case law of the Constitution.
There's no provision forbidding secession either, so they technically very much can. All that's not forbidden is allowed.