I cant think why any grown woman would want to get into the bath with a 4 year old 
It doesn't matter that its MIl - if my own DM had done this I would be asking her - why.
I think there is a difference between being in the bath and DC coming along wondering what your doing etc. But as a granny if I needed to wash with young dc there its more likely I would forgo my wash until later or have a quick shower.
Maybe I am mad maybe its very common? They feed DC off their own forks in spite of being OTT about dirt and cleanliness - its like a religion for mil - but very happy to feed dc off fork shortly after illness..very smooshy with them kissing on lips - FIl is very very physical with them - lying down on grass in summer with DD on top of him after playing .
FOR THE RECORD if I felt anything else was going on of course I would stop them going but my family are just not this physical...DF would give bear hug at greeting and thats it. I really struggle with it - but keep telling myself its OK they are loving GP - but having a bath with DD?
why> why on earth would you want or need to do that?