@NannyOg Well aware as I am late 60s and look after my DGC.
So, if your children decide in another say 8 years to have another child - can you categorically say that to be fair, despite any additional health needs that you may have developed, and despite the additional years that would be taking you to your mid/late 70s, that you'd guarantee that you'd look after any future grandchildren, to the same degree as you've looked after the grandchildren that you currently have? That in 12 years, when that baby becomes a 4 year old, and you'll be in your late 70s/early 80s, you'll happily take responsibility for that 4 year old, take them out an about with 3 other older children in tow too?
Or is there a point, where it's OK to love your grandchildren, but to not want to be in sole charge of high spirited preschoolers, because you simply feel too old to be able to care for them properly, or just too tired to want to.
MIL is not obliged to have sole care for a 4 year old, she just isn't, and if she doesn't feel able to do it properly anymore, then frankly it would be irresponsible for her to do so.
There's no indication from OP that MIL treats them differently apart from this, or that she favours one GC over the others, or even that she doesn't routinely see the younger DD. She just doesn't have her over or take her out, on her own, without parents. And that is because sole responsibility for a 4 year old is hard, whereas sole responsibility for a 12 year old is easy. And MIL, quite reasonably, either does not want to, or does not feel able to take sole responsibility for a 4 year old.
If OP wants her 4 year old to spend more time with MIL, either she or her DH will have to accompany her. It's that simple. I don't understand why the OP has a problem with this?