Head to Greve de Lecq (beach) and your little one will be happy all day playing in the stream and building dams. Lunch at Colleen’s cafe.
Jersey seafaris rib tours are consistently seeing dolphins on their trips.
Historical sites will probably be too challenging for a 4yo, but they are excellent. Get the amphibious vehicle over to Elizabeth castle at high tide.
Good short walks on Grosnez common, with awesome view to the other islands, plus a castle and German military bunkers.
Hire a car to make the most of it and explore all the beaches.
Breakfast at el Tico cafe on St Ouen’s watching the surfers.
If the sun shines Jersey beaches are better than anywhere in the Med.
Not many decent self catering options unfortunately, but you should be ok out of school holidays.