It's up to you. Shikoku is lovely, but I spent several days there in 2012 without encountering another Westerner, so it may be intimidating for a first-time visitor. (I work as a Japanese-English translator, so language is not a problem for me.)

You will have plenty to see just in Kansai (the Kyoto-Osaka area). I'm a big fan of Nara, which is more low-key than Kyoto.