4th generation nuclear does not exist yet. Building a nuclear reactor just with existing tech can easily take a decade. Building a nuclear reactor that has never been built before with a completely new concept will take at least another decade to develop from a concept to a real working thing.
Even in the case that any of the concepts discussed as 4th generation nuclear will work at all (there's a whole number of previous "new nuclear" concepts that have been tried and never went anywhere), it won't matter in the time most crucial for climate mitigation.
Even in the case that any of the concepts discussed as 4th generation nuclear will work at all (there's a whole number of previous "new nuclear" concepts that have been tried and never went anywhere), it won't matter in the time most crucial for climate mitigation.