I can understand if a couple have had, or are having, problems with either set of in-laws they may want to set 'boundaries' (I personally hate that term when we're referring to adults). But, for a family who have had, from what we know of the OP, what sounds like a good relationship, to be excluded from seeing their new grandchild for 4 weeks now, when the other grandparent is there daily, is totally unfair and unreasonable.
Of course, if, as some have suggested, there is some other reason why they don't want them to visit, such as some major problem with the mother or baby, then what son wouldn't explain this to his parents for goodness sake?! And for those who say that perhaps the mother doesn't want anyone to know, I'm sorry, but the paternal grandparents aren't just 'anyone', they're the parents of the son, and grandparents to the child, and they care about them!! They're not just some random people who have no right, or interest in what's happening to their family! And, for those who believe that grandparents should have no rights or interests in the new little family, may be not legally, but whether you like it or not, they most definitely are still family. And for one side of a family to be treated so unfairly, is just downright cruel and selfish.
The OP has mentioned further down the thread that she'd be happy with a 15 minute visit. There was nothing to stop her son or DiL, or indeed the maternal GM, from saying that a short visit would be appreciated for now, as new mum is finding it all very tiring/difficult, or whatever.
One month in, and still no visit, simply shocking! I just hope that the reason the OP hasn't been back for a few days is because she's been busy visiting her new grandchild, and all is well.