Honestly, not only do I see myself returning to P:M after awhile of Smash 4, but even discounting how much time I've played it before Smash 4's release, I think I will be clocking more time into P:M long-term.
Especially if P:M brings back Pichu. My little sister is the only person in my house that plays Smash on a mid-highish level besides me, and she loves playing as Pichu.
On an unrelated note, despite being in middle school, my sister is pretty darn good at Smash. We went to a local vanilla Brawl tournament last year (a tournament in which everyone participating besides her was a high school student) and she got 3rd... and she beat me.