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Join Rob Barrington and Anam Tebha as they show you a hand that could have used conventional help. Puppet Stayman is a useful tool that helps our side discover Major Suit fits after we open No-Trump. Check out how it works in this video.
what is wrong (bad) with responder's bid of 4 diam over opener's reibid of 3 diam when responder holds 4 cards in each major?
They went through that a bit quickly. In "standard" or old-style Puppet (at least over 2N-3C), the answers were:
3D = no 5-card major but at least a 4-card major
3M = 5-card major
3N = no 5- or 4-card majors.
If we play this way, then theoretically responder could bid 4D to show 4-4 in the majors conventionall. However, even in that case, it's a waste of a bid; 4D would often be necessary as a slam try in diamonds.
IF you want to play 3D as showing a 4-card major, then you can still bid with both majors as responder by showing one major and if partner denies it, showing the other one. It's a bit awkward, but it frees up 4D to be a natural slam try.
However, I also agree with Anam&Rob that it's better to have 3D as "no 5-card, may or may not have a 4-card major" because that way defenders will be a bit more in the dark. Dummy will show their hand more, but that is only a factor for the opening lead.