I was just thinking about this today in regards to my MIL! In our home I like to limit the tv to either none or only up to 1 hour a day. Sometimes it does depend on what I have to get done. For instance -schools and preschools were closed for a week here in January due to snow and ice, but my husband and I still had to get some work done from home, so they got to watch a lot more than usual! That also happens sometime during the holidays. However, at my MIL's house, the tvs are ALWAYS on -constantly and my oldest does nothing but watch cartoons the entire time he's there, be it 2 hours, 6 hours or overnight (except for sleeping). The way I view it is, since I know this is going to happen, I don't allow any tv the day before or after. I've told him the reason why. I make a special effort to get both boys out to play, run around or burn off steam in some manner before sending them over there.
My MIL loves to see our boys and she keeps them overnight fairly often during the weekends or she'll watch them for a few hours in the afternoon or evening while we go see a movie. They are a lot for her to handle, and since she offers and it's free, I don't worry so much about it. I remember as a child visiting relatives and grandparents and watching hours of television I was never allowed to watch at home! I think since she's watching him for you and it's only 4 hours a week -I wouldn't take issue with it. Since he is getting such a dose there, don't let him watch at home, or only let him pick one 30 minute show or something he watches every day (except the day he visits her). You say he's in school, so is he reading yet? He could take a book to her house. Other than tv, I remember spending hours reading at relative's houses. Also, if he's old enough to be in school, television still isn't great for him, but it's not as detrimental to development as it would be if she was parking an 18 m old in front of the tv for half a day (and most toddlers wouldn't stay there anyway). As long as he's watching age-appropriate programming, I wouldn't worry so much. Another thing to think about -does he have homework? If he does, let your mom know that he's not to watch tv until his homework is finished. Of course let him know this too!