Correct word for a 5-part piece of art?

The right word is indeed pentaptych.

The ptych part is directly from the Greek πτυχή=fold — usually these things fold on hinges. Penta- is the combining form for "five" in Greek.

There are a number of similar words: diptych (two parts); triptych; tetraptych; pentaptych; hexaptych; heptaptych, and so on. The hypernym is polyptych, (poly=many).

OED does include pentaptych, but marks it as "rare", so it's not really surprising that Google wants to search for Pentateuch instead.

OED also says that the pronunciation is "pentaptic", but I've never heard anything but /-tʃ/ for any of these words, and "pentaptitch" doesn't really show that the "pt" belong together from the Greek πτ. I would prefer a pronunciation of /ˈpɛntəˌtɪtʃ/ "pentatitch" (treating pt- as it is in pterasaur).

I think Polyptych is the more general word … Looking from there you can find names for some specific number of pieces: https://en.wikipedia.org/wiki/Polyptych

Pentaptych is the one with five panels or pieces, it seems…