Ok don't panic
Shingles can not be caught but it sheds Chicken Pox virus so he may catch this
The Health Protection Agency advise that children / adults can go to work / school with Shingles as long as it is covered as the risk of catching it are minimal.
Shingles, unlike chicken pox is contained to one area and usually in a place normally covered my clothes... I would say that with good handwashing, separate towels, no kissing and mum getting changing in her bedroom away from you guys you 'might' escape it.
Are you breast feeding? If so then he might get some immunity.
Is he normally a well child? Does he have any chronic conditions? If he has had steroids in the last month-6wks I would seek advice from a GP ASAP as chicken pox can be very bad for children who have had steroids recently.
4mo is little to get chicken pox and there is no telling how bad he might get it... however the risk of him catching it from your mum is possibly less than if he had come in contact with a child on a bus who was about to erupt in spots.
Plus he's been in contact already - taking him away may not stop him getting it and most children cope well with chicken pox, even this young.
If you are really unsure and for really specific advice I would definitely call NHS direct - they will be able to take a history on your little one and give really sensible advice.