If I insert a Japanese SIM into my Nexus 4, can I have unlimited access to the Japanese Google Play Store? I read this thread (How does Google Play determine the region on a tablet without SIM card) and it seems like the region is determined by IP, but if I insert a SIM purchased in Japan into my Nexus 4, will I be able to purchase Japanese apps, games, books, etc even if I don't have a Japanese credit card, or will I need that as well?
No, having a SIM card from a particular country does not get you that country's Play Store. See this answer on a related question for instructions from Google Play support about moving to a different country.