I visited my parents this weekend who got themselves an R4 Iconic+ a few weeks ago, and had a play about in it. The dimensions on paper don't make it sound much bigger my R5, but in the metal it looks and feels like a noticeably larger, chunkier car. Interior is mostly the same as my R5, there's a bit (and only a bit) more room in the back for passengers though I still wouldn't call it spacious. It feels set up for a slightly softer ride as it was soaking up the poor road surfaces a little better than my R5 does, but otherwise they feel fairly similar to drive. I like the one-pedal drive too, didn't think it would be a big deal but do find myself wishing my R5 had it now (although surprisingly, it doesn't work in Reverse). The Harmon Kardon audio system sounds really good too, again unfortunately not an option on the R5 until very recently.
Gave them both a decent clean and got some photos: