That about says it The question whether to switch from RER B to metro line #4 at Chatelet or Gare du Nord or at ???

I think we've switched at Gare du Nord both times we've done this. Not recalling many steps or long hallways there.

If you want to minimize walking distance, which I guess is the idea, I think Gare du Nord is the best on that score, second choice would be St Michel. Last would be Chatelet/Les Halles.

Thanks. That was my guess too -- although I remember lots of walking at St. Michel also...

There is lots, but not as much as Chatelet/Les Halles, I don't think.

One of the three map sets in "Le Petit Parisien 3 Plans par Arrondissement" overlays streets with Métro and RER lines. Having just measured the correspondences, I'd switch to Line 4 of the Métro at St-Michel. It's the shortest of the three, followed by Gare du Nord - though there doesn't appear to be much difference between these two. Châtelet, however, clearly requires the most steps. While it's been a while, I've taken RER-B from CDG and switched to Line 4 at St-Michel. Even this one can seem a bit long when you're rolling luggage with you.