Can I switch between Silktide 4 and Silktide 5?

Yes. In V5, open the account menu and choose Back to original Silktide to return to V4. Your websites, results, and account configuration are shared - you are not maintaining two separate datasets. A few names differ: what V4 calls tags appear as labels in V5. See What happened to tags?.

Prefer V5 for new capabilities. Use V4 temporarily if you rely on a feature that has not landed in V5 yet. See Introducing Silktide 5.